Continuity / Ohms testing help / Short ?

[king185]

Hi,
I would love some help understanding an issue I’m having on a 16mm SWA cable running from the CU in my house to the CU in the shed.
I believe there is a problem with the cable (possibly between E and N), and I have used a multi-meter to do some checks on the CU (shed side) and below you will see my finding.
Please note that the tests are done whilst the MCB in the house is OFF so there is no voltage at the shed whilst performing these tests, however the cables are still connected at the CU in the house (shed ones are disconnected).


https://1drv.ms/i/s!AicNdL7JWhGahawkCq1gV5yjwZM3uA?e=UoM1WT
https://1drv.ms/i/s!AicNdL7JWhGahawj2CAfR9zxn-Pi5g?e=Obqlme
I think my electric supply coming into my house is TN-S. I will attach a 2 photos above so someone might be able to confirm this.
Also to note i did a continuity test between the earth bar and the neutral bar in my CU in the house and we have a LOUD BEEP.


https://1drv.ms/i/s!AicNdL7JWhGahawcNYe4hV-GacO3cA?e=2mzlCy
Above is common (black cable from multi-meter) connected to N and red cable connected to E.
Multi-meter is in AUTO MODE set to OHMS and we get 2.1 ohms.


https://1drv.ms/i/s!AicNdL7JWhGahawWWt91uVD49sVKiA?e=w6s24B
Above is common (black cable from multi-meter) connected to N and red cable connected to E.
Multi-meter is in IN MANUAL MODE set to OHMS and we get 3.9 ohms.


https://1drv.ms/i/s!AicNdL7JWhGahawbWsXbg4qcmA6VaA?e=ZHEvAA
Above is common (black cable from multi-meter) connected to N and red cable connected to E.
Multi-meter is in CONTINUITY MODE and we get a LOUD BEEP with a reading of 7.9 (is this ohms?) – I do not know what this means.
But I do understand that there is a pathway between N and E which seems like that maybe the fault.


https://1drv.ms/i/s!AicNdL7JWhGahawU1iw4AK0u4_8v0w?e=Tt1fWV
Above is common (black cable from multi-meter) connected to E and red cable connected to N (notice I switched the cables around to see if it makes a difference).
Multi-meter is in IN MANUAL MODE set to OHMS and we get 0.0 ohms
This occurs again even in AUTO MODE (no photo for this) for ohms it gives a straight 0.00 instead of OPEN LINE.
I find this strange, but I don’t know why. Haha.


https://1drv.ms/i/s!AicNdL7JWhGahawS2aPPBwa4Fi_ynA?e=PRyDnv
Above is common (black cable from multi-meter) connected to E and red cable connected to N (notice I switched the cables around to see if it makes a difference).
Multi-meter is in CONTINUITY MODE and we get a LOUD BEEP AGAIN with a reading of 4.5 (is this ohms?) – AGAIN I do not know what this means. But I think something is wrong.


https://1drv.ms/i/s!AicNdL7JWhGahawZYfAcD4saxhHRFg?e=I0nULJ
Above is common (black cable from multi-meter) connected to N and red cable connected to L.
Multi-meter is in AUTO MODE set to OHMS and we get 5.063 M ohms.
I think this is MEGA ohms and I don’t understand if this is normal, I think this is normal.


https://1drv.ms/i/s!AicNdL7JWhGahawY10VSgWlnTrBv2A?e=YIOnj9
Above is common (black cable from multi-meter) connected to N and red cable connected to L.
Multi-meter is in IN MANUAL MODE set to OHMS and we get OPEN LINE.
I don’t understand why there is a difference between AUTO MODE showing over 5 MEGA ohms and this manual mode showing OPEN LINE, but I think this is GOOD and normal.


https://1drv.ms/i/s!AicNdL7JWhGahawaIPdmInzCpcFK1g?e=SGCVQc
Above is common (black cable from multi-meter) connected to N and red cable connected to L.
Multi-meter is in CONTINUITY MODE and we get a NO BEEP with a reading of OPEN LINE.
This looks OK to me.


https://1drv.ms/i/s!AicNdL7JWhGahawXMPHXIuM7a3eoDg?e=H2dJy1
Above is common (black cable from multi-meter) connected to E and red cable connected to L.
Multi-meter is in AUTO MODE set to OHMS and we get 5.99 M ohms.
Again I am not sure if this is normal.


https://1drv.ms/i/s!AicNdL7JWhGahawTQTuFMqUuID58Hg?e=D68YO2
Above is common (black cable from multi-meter) connected to E and red cable connected to L.
Multi-meter is in IN MANUAL MODE set to OHMS and we get OPEN LINE.
I think this is GOOD.


https://1drv.ms/i/s!AicNdL7JWhGahawVl-ZTh4bCmK7H8A?e=XHN1Mq
Above is common (black cable from multi-meter) connected to E and red cable connected to L.
Multi-meter is in CONTINUITY MODE and we get a NO BEEP with a reading of OPEN LINE.
This looks OK to me.


Does anyone have any thoughts as to what is going on?
Do you think I should do further tests or is it simple to say there is a NEUTRAL to EARTH fault on my 16mm2 SWA cable?

Help will be much appreciated.

Kind regards.

 

[nicebutdim]

What was the initial issue that led you to investigate the cable?

I can not see any images attached to your post, but you will find continuity between neutral and earth on a TN-S supply if neutral and earth are connected in the board. Neutral and earth are connected at the supply side and switching off an MCB isolates only the line conductor (assuming single module MCB commonly found in domestic installations).

 

[pc1966]

If you switched off the main supply isolator (i.e. main switch to the CU) then you should not see any N-E connection. You did switch off before testing?

However, if you do see N-E fault that is not proving it is on the SWA, unless you have isolated its ends to measure them separately, as any other circuit (or connected equipment) could have a N-E fault and as said above the MCB only isolate the L side.

 

[oscar21]

Yep, neutral and earth are connected together at the local substation so your multi meter is measuring this path via the house consumer unit. Either switch the house main switch off or take the neutral out of the neutral bar in the house consumer unit and re-test.

 

[king185]

I would like to add some more information here.

I missed some important points. I apologise.

I understand its probably not an E - N fault as i think my supply in my house is TN-S.

The SWA cable is 2 core only L and N with the steel cable around acting as an E.
All the tests ive conducted to earth is to the outer steel cabling.

This 16mm cable was there when the house was bought and runs from the house to the shed and powers another CU in the shed.

The reason for this investigation is that the MCB (running the SWA cable) in the house keeps tripping usually slowly anywhere between 5 mins to 2 hours but sometimes immediately.
Nothing in the shed is faulty this has been tested throughly and each induvidual MCB in the shed has been disconnected.
It all boils down to this cable as the tripping of the MCB in the house still occurs even when the cable (shed end) has nothing attached.

The CU in the house does not have ANY RCD protection. It is an old CU which will be updated in the near future.

I will perform all the tests after the cable has been disconnected from the CU in the house and in the shed. So i will simply be testing a cable that has no connections on either ends.

The multi-meter has a feature where i can zero the leads, before attempting any continuity measurements. This can be done.

If the resistance cannot be tested correctly with this multimeter an insulation resistance tester can be used to check at a later date. But please double confirm this is the case.

OBJECTIVE:

Test the cable correctly.

Dig it out of the ground.

Inspect it all the way along. (this has been mostly done where i can access the cable). There are signs of damage which have been fixed however not all the cable is accessible as some of it goes underground into concrete.

Get a hack saw and cut it where it makes sense. Saving as much cable as possible as its very expensiver per meter.

Perform the same tests to make sure cable going to the house is good.

Cut it again and perform the same tests to make sure cable going to the shed is good.

Get 2 resin underground SWA cable joint kits.

Bury the cable.

My apologies if some tests were not conducted properly. Im learning alot already.

Thank you

 

[timhoward]

I understand its probably not an E - N fault as i think my supply in my house is TN-S.

TN-anything has earth (Terra) joined to Neutral, the only difference is where.
To repeat what others have said - this is why the tests are meaningless unless your main switch is off, interrupting L and N.
Turning off the MCB alone won't do this.

OBJECTIVE:

Test the cable correctly.

The correct test to prove a cable is safe to energise would be insulation resistance testing, and unfortunately a multi-meter doesn't typically have this ability.
That said, if done correctly (as already described) continuity testing might give an indication of the low resistance fault required to trip an MCB. Though if it's the far end of the cable, a normal multimeter might not put enough oomph into the test. (volt drop and low current test may give unreliable readings)

From what you've said I seriously doubt it's a N-E fault causing this. A non RCD protected consumer unit with an MCB wouldn't care about a N to E fault, it would have to be a L-N or L-E fault to cause tripping.

If an MCB is sometimes tripping instantly, then quite a lot of current is flowing and I'd suggest you stop using the circuit until this fault has been diagnosed and resolved.

 

[oscar21]

I could simply be a faulty MCB, I had one recently that I installed 2 years ago, 40A submain MCB tripping off despite no load, everything meggered out ok. Swapped the MCB to a different circuit and the same MCB still tripped, now taking out the cooker instead. Replaced it and all was well. So it does happen.

 

[king185]

Ok thank you all for answering

The house CU has no RCD protection so there is no RCD tripping, as thats impossible.
As for the shed nothing trips there as in no circuits.
So not using the multi-meter to find faults that dont exsist lol

There is no fault in the circuits in the shed.

The fault clearly lies in the SWA cable. I say this because the MCB in the house constantly trips even when the CU is disconnected and removed in the shed.

The MCB that trips in the house has the cable that runs to the shed attached to nothing.

This MCB was already replaced and it did not fix the issue. Not a faulty MCB.

Also just to mention the 16mm SWA cable that runs from the house to the shed is around 55 to 65 meters in length. Its long!

It is possible the cable was damaged somewhere due to foundation of a fence. Either way it cannot all be removed from the ground as some of it goes under the concrete foudations of my neighbours fence.

The middle or some section of the cable will but cut out and replaced, but of course i will get a sparky to look and test with an insulation resistance tester so we know where to cut in order to save as much cable as possible as its expensive!

Thank you all for the help.


https://1drv.ms/i/s!AicNdL7JWhGaha5vj9GjThJPJyp-ZQ?e=lTcdpH
https://1drv.ms/i/s!AicNdL7JWhGaha5wgCWO5_XMwD2-ZA?e=pSjhNw
Above are photos of my house CU

 

[SparkySy]

It is possible the cable was damaged somewhere due to foundation of a fence. Either way it cannot all be removed from the ground as some of it goes under the concrete foudations of my neighbours fence.

My thoughts,

There is a high possibility if there is a fault on that cable that is where it will be.
If the cable has been damaged then water ingress will likely be causing a short possibly live to SWA, hence the differing lengths of time the MCB will stay energized for.

As said before an insulation resistance tester is needed, and the circuit ideally needs safely disconnecting until the fault is properly diagnosed and rectified!

 

[timhoward]

The middle or some section of the cable will but cut out and replaced, but of course i will get a sparky to look and test with an insulation resistance tester so we know where to cut in order to save as much cable as possible as its expensive!

To set expectations, an IR tester will confirm there is a fault on the cable and show the nature of the fault. It unfortunately won't help with the location of it.

Other test gear or approaches will be needed to estimate the fault location. These range from modestly priced Wheatstone bridge based testers which enable old school methods (Varley test), to very expensive modern time delay reflectometers. Being honest, there isn't a high chance that an average domestic sparks would have either (though the latter can be hired).
You're probably going to need to find an above average sparks to help pin-point the fault.
Which part of London are you? There are some geniuses who lurk on here that love challenges....

 

[pc1966]

If the MCB is not tripping immediately then it is not a hard short, so you would need a high voltage insulation test to see the problem (though a multi-meter showing less than 10M at low voltage is suspicious).

Some of the fault-locating techniques such as Varley / Murray loop need a hard fault to work, others like time-domain reflection can work on a fault seen when flashing over at typical operating voltages.

How long is the cable? Was it buried directly in rocky ground rather than having sand around it?

I am asking as 16mm 2C SWA is about £7.50/m inc VAT and hiring fancy reflectrometry test gear, plus the resin joint (or multiple joints if more than one fault) may well cost more than 30+ m of cable, so you might be as well to simply have the cable IR tested and if faulty replace the lot.

With sand around it. Or in twin-wall duct if you want to put a network cable (or better still fibre) or there at the same time. If buying sand then look at bulk delivery from a quarry or merchant, works out way cheaper than bags of it.

 

[king185]

To set expectations, an IR tester will confirm there is a fault on the cable and show the nature of the fault. It unfortunately won't help with the location of it.

Other test gear or approaches will be needed to estimate the fault location. These range from modestly priced Wheatstone bridge based testers which enable old school methods (Varley test), to very expensive modern time delay reflectometers. Being honest, there isn't a high chance that an average domestic sparks would have either (though the latter can be hired).
You're probably going to need to find an above average sparks to help pin-point the fault.
Which part of London are you? There are some geniuses who lurk on here that love challenges....

Near heathrow buddy.

 

[king185]

If the MCB is not tripping immediately then it is not a hard short, so you would need a high voltage insulation test to see the problem (though a multi-meter showing less than 10M at low voltage is suspicious).

Some of the fault-locating techniques such as Varley / Murray loop need a hard fault to work, others like time-domain reflection can work on a fault seen when flashing over at typical operating voltages.

How long is the cable? Was it buried directly in rocky ground rather than having sand around it?

I am asking as 16mm 2C SWA is about £7.50/m inc VAT and hiring fancy reflectrometry test gear, plus the resin joint (or multiple joints if more than one fault) may well cost more than 30+ m of cable, so you might be as well to simply have the cable IR tested and if faulty replace the lot.

With sand around it. Or in twin-wall duct if you want to put a network cable (or better still fibre) or there at the same time. If buying sand then look at bulk delivery from a quarry or merchant, works out way cheaper than bags of it.

Cable from house to shed is approx 60m. Very expensive to replace. So i think it makes sense to do some test with an insulation resistance tester and cut and test and keep repeating till we have two good pieces of cable then i can purchase the length of the gap and it can be joined.
cable is buried in the garden just under the grass and soil, some parts go under the conrete base of my neighbours fence.

Thanks

 

[Other_Tucker]

Check the cable first, disconnect, really disconnect both ends so they're pointing out into free space, both ends. Check isolation between conductor 1+2, 1+3, 2+3. Ideally the multimeter should read >200MOhms. Run out a long lead and test 1-1, 2-2, 3-3 they should be as expected and similar.

If it fails either open or shorts tests then run your fingers literally along it, all the way that's above ground, you'll see the slice or kink or crush mark. If you have a dead short you're in luck, 1.42 Ohm from one end and 4.26 Ohm from the other is exactly 0.25 * the way along, normally where someone's been digging or a fence post. We only used Time Domain Reflexometers if say a buried camera cable was open circuit hundreds of metres up a racetrack.

Your multimeter should be fine if you didn't buy a stupid auto-ranging one, I was an electronics designer and used every function of my cheap one, a https://www.distrelec.de/en/digital-multimeter-250v-200mohm-uni-ut131d/p/30158662 while other broadcast engineers spent £££s on Fluke's they never understood how to use.

 

[king185]

Wow, thanks for the advice. Alot of people seem to find a multimeter hilarious for resistance checks to find an issue in my situation.
I will try and get this checked. Although mine is auto ranging, it can also be set into manual mode. Thanks again

 

[king185]

Some more information.

I probably dont need 100 amps with a 1000v cable at the moment, but i want to replace like for like, and future proof it, also if i ever get the shed extended or build a back room in the garden its good to have the correct cabling.


I dont want to replace the whole cable as alot of the cable goes out and around the house from the front then comes out at the rear in the corner of the house then makes its way down into the ground and covers a distance of about 20-35m meters to the shed.
The plan is to cut it before it goes into the ground and replace approx between 25-35m worth.
All the cable under the ground will be replaced and that should do the trick, and of course it will be tested before its joined up.

The cable was previously damaged only on the outer sheath and a little bit of the earth steel armour was exposed.
This then got patched up with insulation tape and duck tape on top.
(this damaged part will be replaced)

Possible damp ingress and corrosion cannot be ruled out.

The last thing we want is to running around in circles, that is why i am willing to have the underground bit (25-35m) chopped off and replaced!
Surely that will fix the problem.

Also the cable i currently have is 2 core with the armoured steel earth. I will use the same type of cable again, so i will not be spending extra money to buy a 3 core as that is unnecessary.

I can source the cable and joint connectors myself.

Any ideas of how much this should cost and electrician to do? Also how long would it take?

Thanks