Continuity test after Ring Circuit

[Neptune]

After completing a new RFC or modifying an existing one, I will always test continuity across all 3 conductors at the Consumer unit. I use the continuity function on my Fluke 2 pole tester to do this. This is done whilst the circuit is dead.

Prior to this, I am very diligent with the conductors in each socket terminal and ensuring that I have the correct amount of cable length. I also screw the terminals tight again once the excess cable is bent in the back box and before I screw in the socket plate incase anything had come loose.

This means that when it comes to the continuity test, everything is okay. Else, I will revisit each socket, rectify and test again…

I know “proper” electricians will do R1, etc. tests using a low ohm meter. What am I missing without doing these tests and can any of these be done without buying the very expensive low ohm meters that electricians have?

I appreciate that the very simple response is that these test have to be done.

Thanks in advance.

 

[buzzlightyear]

The purpose of a low hom meter is to test a ring or other to see what values you have got.

 

[Neptune]

The purpose of a low hom meter is to test a ring or other to see what values you have got.

Thanks but can you elaborate on this please. What am I practically missing from my approach versus using this meter?
Also, are there any alternatives to the very expensive low ohms meters?

 

[westward10]

The short circuit current for low ohm resistance testing should be 200ma.

 

[Electro-tech]

Without R1+R2 ohm values, how will you know whether your overcurrent device will function under fault conditions, or have you chosen the correct one.

 

[Eddy Currents]

What am I missing without doing these tests

Basically you are confirming continuity of the ring and skipping all of the other tests which are required to confirm the circuit is safe to energise and use and will disconnect in the required times in the event of a fault

MFTs do all the required tests not just low ohm readings. R1+R2 will confirm earth at every point and also polarity. Values are recorded and checked against tables in the regulations to ensure they meet disconnection times but other tests like insulation resistance should be confirmed

Also how are you testing and recording values for a new RFC?

 

[Dartlec]

The two separate tests for a RFC that use a low resistance Ohmmeter are to check slightly different things.

The initial continuity one is to ensure that there is a "ring", which any continuity tester can show to some extent.

However, the point of the low resistance Ohmmeter is to get actual values for r1 (Line), rn (Neutral) and r2 (CPC)

The values for Line and Neutral should be consistent with the known or estimated length of the circuit, and also very similar to each other (<0.05 difference generally). That ensures that the ring is consistent with no strange loops. It also verifies that the terminations are satisfactory and not introducing resistance.

The CPC resistance should be higher, relative to the smaller size of the CPC compared to Line and Neutral (1.67ish with standard 2.5 twin and earth)

A 2 pole tester that shows continuity will give you no idea of how good the connection is, and wouldn't show up, for example, a poor loose connection on one leg of the ring.

That may be avoided if you are installing the circuit from scratch and know exactly where all the connections are, but on an existing circuit there may well be hidden junctions boxes, or connections that are not immediately visible.

Then there is R1 + R2 which is tested at each point, and is ensuring that the earth fault loop resistance is low enough that the installed protective device (mcb usually) will trip if an earth fault occurs. The tables give maximum readings for Zs, which is the combination of R1+R2 and the external earth reading Ze, though Zs can be tested directly too with a multifunction tester, or a separate earth fault loop tester.

So there is a reason behind the requirement to test and get proper readings, even before the final reason that a certificate when filled out needs the values to be inserted, to assist future electricians who might be troubleshooting the work.

These days, there are some older Multi Function Testers going quite cheaply on Ebay and the like. Something like the MFT1552 is almost bullet proof and often costs not much more than £200. That will still do every test you might need to do in a domestic setting and really is a sensible investment for anyone doing more than a one off job, even if it's just replacing the odd socket.

 

[buzzlightyear]

Using the low reading ohm meter, test between Line and Earth on every outlet on the ring circuit. The highest reading is the (R1+R2) for the circuit. The readings at each of the sockets wired on the ring should be very similar and the value should be according to this formula: (r1 +r2)/4.

 

[Neptune]

A 2 pole tester that shows continuity will give you no idea of how good the connection is, and wouldn't show up, for example, a poor loose connection on one leg of the ring.

That may be avoided if you are installing the circuit from scratch and know exactly where all the connections are, but on an existing circuit there may well be hidden junctions boxes, or connections that are not immediately visible.

With a new circuit - and perhaps naively - I know that each connection has been correctly inserted and the ring follows the correct format. The continuity test further validated that we have a “circuit”. I’ve assumed that is adequate.
Realistically, where are the additional tests likely to pull out issues in this scenario? I appreciate that existing RFC’s May have other lurking issues that such tests draw out.

Something like the MFT1552 is almost bullet proof and often costs not much more than £200. That will still do every test you might need to do in a domestic setting and really is a sensible investment for anyone doing more than a one off job, even if it's just replacing the odd socket.

Will this do the insulation resistance test too?
Also, do these devices need regular calibration and have to be sent off to a specialist for this purpose?

 

[timhoward]

Great answers already.
In short and sweet terms you are missing checking for
-loose connections
-enough fault current will flow to trip the protective device
-accurately checking ring end-to-ends and relative length of CPC
-you haven’t accidentally created a loop in the ring

I’d assume this means you also aren’t doing IR tests so you aren’t checking for damaged or snagged cables adequately either.

I’d also assume you aren’t able to test the mandatory RCD is working for the new sockets circuit.

I know it’s painful but you are at the point you need an MFT. You could buy cheap separate units but I’d say it’s better to get an old 1553
(Or maybe a Kewtech KT53 as it’s easy to use and a decent first tester)

 

[Dartlec]

Will this do the insulation resistance test too?
Also, do these devices need regular calibration and have to be sent off to a specialist for this purpose?

Yes any of the MFTs will also do Insulation Resistance as well as RCD testing of the sort needed in domestic (aside from car EV chargers, which is a different specialised segment.

Occasional third party calibration (usually is in the £40-60ish price range) is ideal, but not always necessary Something like a Calcard will allow ongoing verification of the results for continuity and insulation. Zs (Earth Loop) and RCD test can be checked against a known circuit to check for any drift over time.

To be honest with most MFTs, issues with reading errors are almost always down to the leads which are easy to replace.

 

[Dartlec]

With a new circuit - and perhaps naively - I know that each connection has been correctly inserted and the ring follows the correct format. The continuity test further validated that we have a “circuit”. I’ve assumed that is adequate.
Realistically, where are the additional tests likely to pull out issues in this scenario? I appreciate that existing RFC’s May have other lurking issues that such tests draw out.

If everything is installed perfectly from new then the tests are really just a way of proving that you've done the due diligence.

In practise, unless the cabling is all visible from start to finish, there is always the opportunity for things to not be what they seem, and the tests are designed to ensure that the installation is safe to energise/re-energise.

Even with a new install of cable there are multiple things that could go wrong:

Has the plasterer nicked a cable with his trowel, have the skirting board installers put a nail through a cable, has the home owner fitted a picture right where a cable runs down a wall (perhaps from the other side of the wall without realising), was the cable manufactured with a fault, or was it sold as genuine cable when it was actually cheap, non compliant cable.

None of those would necessarily prevent the installation from working, but they may well mean that it is dangerous to leave powered.

With an existing RFC then there are the hidden junction boxes under floors and cables that have been chewed by mice to consider, or perhaps a section of older rubber 50s cabling left in place that is starting to deteriorate.

The main benefit of a proper test instrument and doing the tests is that you know you've done the work safely and can prove it. The certificate is as much for your protection as it is for anything else. Should a fault develop a month later due to other work, being able to show that the readings were correct gives you a level of immunity from potential comebacks. Not to mention if something more serious was to happen, you could prove to any investigation that you have met your legal obligations.

 

[Neptune]

Thanks for all the responses. I am up for purchasing the meter and as someone said, I think I have now reached the stage where this will be necessary. I may have some more questions on the precise model that I am intending to purchase.

In the meantime, I have a ring main issue that you may be able to help me troubleshoot please. Here is some background:

• I installed two separate ring mains. They are both on the same RCD and wired into a 32amp MCB
• Both have continuity across all 3 conductors when tested with my 2 pole tester. As you know, I don't currently own a low ohm meter
• 1 of these ring mains works fine and we have been using it for a couple of weeks
• The other ring main, when energized, tested successfully using the plug in tester across all sockets but when I plug a bosch hammer drill (as an example), it trips the RCD. This happened in all of the sockets I tested.
• The neutral for this ring is on the terminal bar for the correct RCD
• As a further test, I wired (Live into the MCB on the other side and Neutrals into the associated terminal) the affected ring onto the other RCD (I have a dual RCD CU) and again socket tester is fine. It lights up to indicate that everything is okay but as soon as I plug in my drill (or a kettle and even a phone charger), it trips.
• I then reverted it back to the original RCD and same issue remains.
• Leaving these wires connected was causing the other circuits to misbehave. As a example, the heating thermostats were happily switched on but when we interacted with them, they were also causing the RCD to trip. When I unwired the affected Ring, the thermostats worked fine.

I am hoping the above provides some clues as to where the problem may lie and how I troubleshoot it. One approach may be to have a floating wire which returns to the consumer unit as one end of the ring and I connect this to each socket at a time. Starting with the first socket in the ring and testing this with the drill (or anything I suspect!) as I go along.

Over to the experts.

 

[timhoward]

The other ring main, when energized, tested successfully using the plug in tester across all sockets but when I plug a bosch hammer drill (as an example), it trips the RCD. This happened in all of the sockets I tested.

This is classic behaviour for higher a resistance fault or maybe a Neutral - Earth fault.
To the best of your ability you have proved that each conductor has end to end continuity.
A 2 pole tester will only report continuity of up to about 200 Kohms and will use a low test voltage to do it.
I'm assuming your 2 pole is NOT saying that there is continuity between any of the conductors?

To diagnose this further you urgently need a continuity and insulation resistance tester which can test using up to 1000V and measure from hundredths of an ohm to hundreds of megaohms.

Are you feeling lucky? This powers up, if it works it is an excellent tester that would transform your life!

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or

ROBIN Insulation Continuity Tester KMP 3050DL | eBay

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As to your fault, my best guess is a nail used for capping has missed and has damaged a cable. Or a screw at a socket has caught the Neutral.
Breaking down the ring, with an IR tester is the only way to proceed I'm afraid.

 

[timhoward]

Here's a set of separates, IR/Continuity, Loop, and RCD
The loop tester won't work with RCBOs but you can at least calculate Zs.

ROBIN LOOP TESTER | eBay

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[Neptune]

Would a short circuit not immediately trip when energised? Also, why it it happy with the load of the plug in socket tester and not the drill?
Thanks for your help.

 

[westward10]

You have many 'how do I do this threads' so is this linked to this property or somewhere else.

Garage electrics from main house

We have a garage attached to the property and are having it converted to an office with a small toilet. Electrics will include, internal lights, an external light, a few sockets and a extractor fan. There is a cable coming into the garage currently from the house and is connected into the main...

www.electriciansforums.net

 

[timhoward]

Would a short circuit not immediately trip when energised?

The MCB will trip if enough current flows. If there was a L-N fault it would depend on the resistance. For example if the fault was in the region of 4 ohms it would be pulling 57.5 amps and trip the 32 amp MCB.

The RCD will trip if a very small amount of current 'leaks' to earth.
If there was a N-E fault then the fault is only introduced when a load plugged in.

The plug in tester won't draw enough current to upset either device.
The drill is pulling considerable current.

I'm pleasantly dosed up on wine so I hope that makes some sense.

 

[timhoward]

You have many 'how do I do this threads' so is this linked to this property or somewhere else.

Garage electrics from main house

We have a garage attached to the property and are having it converted to an office with a small toilet. Electrics will include, internal lights, an external light, a few sockets and a extractor fan. There is a cable coming into the garage currently from the house and is connected into the main...

www.electriciansforums.net

Has the OP had the DIY badge all along? If so my eyes need testing and I think I was mixing him up with a different trainee.

@Neptune you've got more out of me that you normally would, and as you've perfectly demonstrated adding circuits yourself and not testing them before energising them is not safe. If it had been a metal appliance and not a drill then you or someone could be getting unpleasant shocks. The RCD is saving the day and these can stick.
Time to get some help in.

 

[Neptune]

Also, could I not split the ring into 2 radials and then test my drill to see which radial it trips? I could gradually increase the “good” radial until it reaches the problematic part of the circuit.

 

[Neptune]

Has the OP had the DIY badge all along? If so my eyes need testing and I think I was mixing him up with a different trainee.

@Neptune you've got more out of me that you normally would, and as you've perfectly demonstrated adding circuits yourself and not testing them before energising them is not safe. If it had been a metal appliance and not a drill then you or someone could be getting unpleasant shocks. The RCD is saving the day and these can stick.
Time to get some help in.

I am planning to purchase a tester for these types of situations. That’s the advice I had n an earlier post but wanted to share this particular issue to see what advice there may be.

 

[Baddegg]

Also, could I not split the ring into 2 radials and then test my drill to see which radial it trips? I could gradually increase the “good” radial until it reaches the problematic part of the circuit.

What you are proposing mate is called bang testing, you will eventually damage the circuit protective device to a point where it may not trip and then you’ll be in a world of pain, you need some proper testing equipment and someone to help you…..

 

[Neptune]

Is the MFT 1553 the ultimate choice for me? I can see that Tim’s suggested a few devices in his recent post. I am happy to pay a bit more for something that will serve me well.

With this tester, is it the initial R readings that would identify such an issue? Would I then Plug this tester at each socket to determine which section of the ring the issue is at?

I would appreciate some guidance around this please. Thanks for your help.

 

[timhoward]

The 1553 is definitely a good solid capable all round tester. It will do all dead tests (continuity and IR) and live tests (earth loop and RCD) to confirm something is fit for use.
Electrical testing is a complex subject and even new sparks don’t learn it initially. It’s not something a quick forum post can teach and must be done with great care. John Ward has made some good YouTube videos on the subject.

It’s not as simple as plugging into sockets, you’ll have to break the circuit down, initially into two and then see which leg has the fault and then continue tracking it down. It will be insulation resistance tests that are needed.

 

[timhoward]

You might want to look at this thread

Basic Testing Procedures

Safe isolation The procedure for proving dead should be by use of a test lamp or two pole voltage detector as recommended in HSE Guidance Note GS38. Non-contact voltage indicators (voltage sticks) and multi-meters should not be used. The test instrument should be proved to be working on a known...

www.electriciansforums.net

 

[IzzyS]

What am I missing without doing these tests and can any of these be done without buying the very expensive low ohm meters that electricians have?

The current range of Fluke 2 pole testers have a continuity range of 0 - 400kOhms. They are worse than useless for continuity checks on domestic wiring. At one end of the scale you could have a perfect connection, at the other you could have 2 (30A, 2.5mm2) radials connected by a 40kOhm resistance and you won't know the difference.

Specifications here

Fluke Two-pole Voltage and Continuity Testers

Fluke Two-pole Voltage and Continuity Testers give you more ways to get results, a redesigned cable assembly, and Fluke’s longest two-pole warranty ever.

www.fluke.com

 

[Eddy Currents]

In the meantime, I have a ring main issue that you may be able to help me troubleshoot please. Here is some background:

This isn't surprising to me I'm sorry to say just by your previous questions. It seems you are dong a lot of electrical work which in my opinion is well beyond the limits of DIY and installing as you have and hoping for the best without doing the correct safety tests is a recipe for disaster. You are lucky that you only have some faults and not caused a fire or worse

I really wouldn't recommend buying an MFT because without the correct training which will give you the knowledge to understand the results and if they are acceptable and what to do about it if they aren't, then you are just wasting your money

Trying to install electrical systems step by step through a forum isn't a safe way to work with electricity and by your many questions it is obvious you don't have the knowledge to do it safely

Do you realise that installing new RFCs is notifiable work? How are you or who is notifying?

 

[Petej999]

What you are proposing mate is called bang testing, you will eventually damage the circuit protective device to a point where it may not trip and then you’ll be in a world of pain, you need some proper testing equipment and someone to help you…..

Don't sink that low Mate, test properly, too many Cisco Kids pretending to be Electricians don't become one please

 

[timhoward]

For less than the price of an MFT he could have skills, knowledge and experience on site for at least a morning. I'd bet most would find and fix his issue within 2 hours.
(Thinking about how long it takes to become comfortable testing, I agree it isn't logical to buy an MFT. I also can't imagine him not forging ahead though, and after an internal argument I thought it best he at least had access to some information about testing)

 

[Neptune]

Thanks for the many helpful responses. I really appreciate it.

I’ve had to halve a ring previously to locate a break in continuity. I am famously with the concept.

I also have a plan for my testing and would welcome your constructive feedback please.

After purchasing an appropriate tester, I am ready to commence the testing. Properly! The MFT is my likely choice from what I’ve been reading above. Again, happy to pay a bit more if it provides some future proofing. Please let me know if you have any further recommendations.

1. Continuity / end-to-end tests
Whilst I thought that my earlier Fluke 2 pole tests were conclusive, @IzzyS post shows that it’s far from it!
This will be a dead test and I am familiar with the safe isolation process.
I will locate the 6 wires for my ring. I will initially do this with the ‘good’ ring and then the ‘bad’ one. I will also ensure that everything is unplugged and the FCU is bypassed.
Here I plan to do the r1, rn and r2 tests. On the “good” ring I expect the r1 and r2 to be the same (Within 0.05) and the r2 will be 1.7 times higher due to the higher resistance of the smaller CSA. The readings for r1 and rn should be less than 1 ohm and determined by the length of cable in the ring. I don’t anticipate any issues with the “good” ring but when I do this with the “bad” ring, I think I will get a different reading for r1 and rn/r2. Even after taking into account the expected uplift for r2. The problematic reading shouldn’t be an open circuit/infinity as the fluke tester detected a closed circuit but it’s probably going to be a high reading. I think this indicates a short circuit or a loose connection?
I should now be in fault finding mode…
My plan is to split the ring into 2 radials by breaking the ring (cables disconnected and hanging mid air for the mid socket in ring) and on the CU end will have the first leg of the (now) radial in terminal block. I can then test at the break point of the ring and see if I get a high/low reading. If this leg is fine, I move on to the other and repeat the process. This should have a fault (unless I am unlucky to have multiple faults!). This is where I will need to continue halving until I’ve narrowed down where the problem is. I can then inspect the back of the socket or even try bypassing this section to see what the overall r readings look like.
I would appreciate your comments on the above as I’ve got my head wrapped around this approach if it’s the most suitable way to proceed.

2. Once the above is fixed, I am ready for the cross connection tests: R1+Rn and R1+R2
The expected results can be calculated from the above r readings.
This is where we create the figure of 8 across different wires of each leg. L from one side connected to N of the other side and the same for the other legs of the ring. This creates an infinite loop. I can then measure resistance at all and any point and it should be the same. Unless it’s a spur.

3. Repeat the above for L and CPC

I am not anticipating issues with 2 and 3 as I’ve checked the route of the ring using a wonder lead and my fluke continuity function.

My main issue is likely to be the first tests. I think…

Thanks in advance.

 

[timhoward]

I’m not going to comment line by line on your exposition, but will simply say that the continuity tests need doing, but are less likely to help you fault find.
Insulation Resistance tests will more than likely be where you can see evidence of the fault and track it down.

 

[Petej999]

The purpose of a low hom meter is to test a ring or other to see what values you have got.

Mr HOM is a bloke in Star Trek isn't he? big tall geezer always carrying suit cases

 

[telectrix]

morning pete. not seen youmuch. how's life?

 

[buzzlightyear]

Me too, how's life Pete. Still gambling on
tools tart. Just bought a nice new Stanley
Tool box too throw my crap in.
There is nothing like good tool to drool
On like a kid in a candy shop.

 

[Petej999]

morning pete. not seen youmuch. how's life?

Fine thanks Tel, you?

 

[Neptune]

I’m not going to comment line by line on your exposition, but will simply say that the continuity tests need doing, but are less likely to help you fault find.
Insulation Resistance tests will more than likely be where you can see evidence of the fault and track it down.

Am I right in thinking that the continuity tests are a prerequisite for the IR tests? Also, if we have a screw or something of that ilk smashed into one of the conductors, would you not expect the continuity test to provide a lower (perhaps a 0?) reading? In any event the live conductors would have different readings and hence indicate an issue. That's what I was anticipating.

On IR testing, if we have a short, I am expecting a reading of less that 1 (probably 0?) and that's when I move in to the halving method to locate the issue. Is that the general approach?

Thanks for your help.

 

[timhoward]

Am I right in thinking that the continuity tests are a prerequisite for the IR tests?

Doing things in the right order has long gone out of the window for your install!
You are right that it would be normal to do continuity tests first. Whether or not any IR testing is also done at first fix, IR testing must be done when the installation is 'assembled' to pick up any damaged / trapped / snagged wires.

Also, if we have a screw or something of that ilk smashed into one of the conductors, would you not expect the continuity test to provide a lower (perhaps a 0?) reading?

Maybe but this will vary according to the nature of the fault. Hence why I'd personally dive straight to the test that will definitely show it up.

On IR testing, if we have a short, I am expecting a reading of less that 1 (probably 0?) and that's when I move in to the halving method to locate the issue. Is that the general approach?

Yes. A healthy new cable should be hundreds or even thousands of Mohms.

 

[Neptune]

Insulation Resistance tests will more than likely be where you can see evidence of the fault and track it down.

A few more questions on this please Tim:

• Can I remove USB sockets with 32amp junction boxes for the purposes of the IR test?
• Do FCU's that are on the ring and not a spur need to be removed? The neon is on load and can be switched off
• If we have continuity as part of earlier tests, when doing the IR test for L and E (or the other conductors), does it matter which leg of the ring you use the L and E from? I assume not but was mindful of the other end of the leg hanging mid air at this point:

- If there are failures, we can use the halving method and I can gradually increase the working leg of the ring to see where the failure appears. At this point, I could either replace the cable or convert the ring into two radials powered by a 20amp breaker. Is that a valid approach?

 

[westward10]

This is not a step by step guide to testing. Testing should be carried out by competent persons or by persons under supervision of a competent person. I have no idea if this is linked to your other threads which follow the same theme of step by step guidance.

 

[Neptune]

This is not a step by step guide to testing. Testing should be carried out by competent persons or by persons under supervision of a competent person. I have no idea if this is linked to your other threads which follow the same theme of step by step guidance.

Wasn’t asking for a step by step guide to be honest. Just the above questions in addition to the detail and understanding I’ve already set out. I’m not an expert but comfortable with my understanding and the fact that this is a dead test.

 

[westward10]

When undertaking any test whether electrical or not you must understand how the test equipment operates and how to interpret the results it gives you, you do not understand either of these from the questions you are asking.

 

[Neptune]

A few more questions on this please Tim:

• Can I remove USB sockets with 32amp junction boxes for the purposes of the IR test?
• Do FCU's that are on the ring and not a spur need to be removed? The neon is on load and can be switched off
• If we have continuity as part of earlier tests, when doing the IR test for L and E (or the other conductors), does it matter which leg of the ring you use the L and E from? I assume not but was mindful of the other end of the leg hanging mid air at this point:

View attachment 98571
- If there are failures, we can use the halving method and I can gradually increase the working leg of the ring to see where the failure appears. At this point, I could either replace the cable or convert the ring into two radials powered by a 20amp breaker. Is that a valid approach?

I would really appreciate any direction that could be provided on the above please.
Thanks in advance

 

[westward10]

Is this related to your other threads.

 

[Neptune]

As to your fault, my best guess is a nail used for capping has missed and has damaged a cable. Or a screw at a socket has caught the Neutral.
Breaking down the ring, with an IR tester is the only way to proceed I'm afraid.

This is precisely what happened! I wanted to share my experiences.
I used my newly purchased Megger MFT to run the continuity, crossover tests and then the insulation tests. The IR test failed.
I halved the ring and identified the good and bad legs. I then kept adding sockets to the good leg until it turned bad. Hope that's making sense...

That's when I noticed that a neutral wire was nicked behind an electrical screw of the face plate. The Megger MFT and halving method made this light work and I enjoyed the experience of troubleshooting it.

• I didn't remove USB sockets and they are fine. I started with a 240v IR test and then increased it to 500v. Although, the latter probably wasn't required as fault showed on the 240v test.
• The FCU on ring remained in placed with switch in off position
• I was able to use either L or N as there was continuity all the way around the Ring before I started the IR test.

My only question is why this fault only caused the RCD to trip under load and a certain amount of load i.e. phone charger plugged in seemed okay but the hammer drill would immediately trip it?

Thanks for your support with this.

 

[timhoward]

I'm glad you found the issue.

My only question is why this fault only caused the RCD to trip under load and a certain amount of load i.e. phone charger plugged in seemed okay but the hammer drill would immediately trip it?

An RCD works essentially on the principle that what goes out must come back, and if more than a certain amount is 'missing' it trips.
So no load at all, nothing, goes out, nothing goes back.
A phone charger is a very low load, and the "missing" amount wouldn't be high enough.
A drill pulls enough current that enough goes the 'wrong way back' (via the cpc/earth ) that the RCD would have an aha moment.

 

[Electro-tech]

As timhoward mentions, it is load related with an imbalance, which might be telling you your drill is on the way out. There is a lot of information regarding how RCD's Work online with pictures too.

 

[Neptune]

might be telling you your drill is on the way out.

I don’t follow this part. I can understand that the drill is drawing current which is then not returning via N, hence the trip.

 

[timhoward]

I don’t follow this part. I can understand that the drill is drawing current which is then not returning via N, hence the trip.

I'm not sure he realised there was a N-E fault at a socket.

 

[Electro-tech]

i.e. phone charger plugged in seemed okay but the hammer drill would immediately trip it?

Apologies having a senior moment should have read the whole senario again.