Insulation Resistance testing method

[Bobby34]

Good afternoon, apologies if this has been covered before i've not long joined so gradually finding my way around.

I have my Part P assessment coming up on a new installation and would like to hear some opinions on how people carry out their IR tests.

In guidance note 3 it gives an example of testing the whole installation in one go (having removed sensitive equipment) using the line at the main switch (repeating the process for 2 way lighting)

My question is what are peoples thoughts on the MCB's and RCD's because unless i'm missing something glaringly obvious don't they need to be up so the voltage then passes through the MCB's and RCD's into the circuits? and if so would doing it like that potentially damage them?

I'm thinking to use the Line at the top of each mcb in turn for the assessment but not sure if thats being over cautious?

Any opinions/thoughts/experience would be much appreciated.

 

[Pete999]

Good afternoon, apologies if this has been covered before i've not long joined so gradually finding my way around.

I have my Part P assessment coming up on a new installation and would like to hear some opinions on how people carry out their IR tests.

In guidance note 3 it gives an example of testing the whole installation in one go (having removed sensitive equipment) using the line at the main switch (repeating the process for 2 way lighting)

My question is what are peoples thoughts on the MCB's and RCD's because unless i'm missing something glaringly obvious don't they need to be up so the voltage then passes through the MCB's and RCD's into the circuits? and if so would doing it like that potentially damage them?

I'm thinking to use the Line at the top of each mcb in turn for the assessment but not sure if thats being over cautious?

Any opinions/thoughts/experience would be much appreciated.

Have a look at this vid, there are plenty more on YouTube that are equally as good.

 

[Bobby34]

Hi thanks for your link but that doesn't seem to answer my question unfortunately it's what peoples views are about testing IR through the RCD's and MCB's that i'm wondering. Also on that video he says to turn the lights off? I presume this is because of bulbs left in but isn't the L and N connected together and tested to earth better for this and sensitive equipment? My question is about a new installation with no loads applied.

Thanks in advance

 

[tlangdon12]

GSH Electrical at Tresham College have recently covered the subject on YouTube. Have a look at

The presenter shows that the test being performed between Live & Neutral conductors connected together and the Earth, and again between the Live and Neutral conductors (and with the switches in various positions to ensure all conductors are tested). When testing, the test probes are applied to the outgoing terminals on the circuit protective devices (MCBs/RCBOs), and the devices are switched off so that the test voltage doesn't pass through the devices.

 

[Bobby34]

Tlangdon12 many thanks that is a great video and a handy tip with the croc clip to test L + N to Earth, I think thats the way I will do it for the assessment but still a little confused as I was taught on my course to do the whole board from the main switch with the MCB's and RCD's up and it also shows it like that in Guidance note 3 but I guess it's better to spend an extra couple of minutes rather than risk damaging the breakers.

Thanks again

 

[mhar]

If it is a new circuit (you say new installation) you must follow the steps for initial verification. You test between all live conductors and live conductors and earth (with the cpcs all connected to the installation met).

 

[Bobby34]

Hi Mhar thanks for the reply, yes I understand however you can either test at the outgoing line of the mcb with the mcb "off" (testing just that circuit) or at the outgoing line on the main switch with the mcb's "on" testing a group of circuits.

Some people like the presenter on the video above test without the mcb in play and others (like the way I was taught and mentioned in Guidance Note 3) test from the main switch (usually I guess on a complete new installation) which tests the whole supply.

Just wondering how people do it that have experience, thanks.

 

[Risteard]

Good afternoon, apologies if this has been covered before i've not long joined so gradually finding my way around.

I have my Part P assessment coming up on a new installation and would like to hear some opinions on how people carry out their IR tests.

In guidance note 3 it gives an example of testing the whole installation in one go (having removed sensitive equipment) using the line at the main switch (repeating the process for 2 way lighting)

My question is what are peoples thoughts on the MCB's and RCD's because unless i'm missing something glaringly obvious don't they need to be up so the voltage then passes through the MCB's and RCD's into the circuits? and if so would doing it like that potentially damage them?

I'm thinking to use the Line at the top of each mcb in turn for the assessment but not sure if thats being over cautious?

Any opinions/thoughts/experience would be much appreciated.

All protective devices must be closed so that the entire installation is in circuit. All switches must be closed throughout the installation. (Also don't forget repeat test with two-way switching.)

It should be borne in mind that the minimum values of insulation resistance required by BS7671 are for the installation (or distribution boards with all final circuits connected for larger installations) and therefore it can be said that testing the whole installation in parallel is the best way to conduct the test. If you don't then you need to calculate the effect of resistances in parallel (1/Rt = 1/R1 + 1/R2 + 1/R3 etc.) to verify compliance with the Regulations.

 

[JAWS]

Tlangdon12 many thanks that is a great video and a handy tip with the croc clip to test L + N to Earth, I think thats the way I will do it for the assessment but still a little confused as I was taught on my course to do the whole board from the main switch with the MCB's and RCD's up and it also shows it like that in Guidance note 3 but I guess it's better to spend an extra couple of minutes rather than risk damaging the breakers.

Thanks again

All MCB's should be on the ON position.

 

[Ian1981]

L-N testing on a more complex installation is a bitch and is advisable to do it in stages before any LED drivers etc are fitted and fittings are in place.

 

[Bobby34]

All MCB's should be on the ON position.

Massive1 that video is good thanks but it doesn't say the mcbs MUST be on just says that it's possible to test the installation in as a whole, it also says about the rccb could be damaged if you put 500 volts through it so it shows the individual way too, I think with everything i've seen I will do it the individual way for the assessment as it seems to certainly be an accepted method and wait to be corrected by the assessor haha, many thanks to the replies.

 

[Bellendian]

I do split boards half and half, so RCD's off, MCBs on, test from busbars to earth and neutral bars.
Rcbos I test ccts individually with conductors removed from unit, half the time probes can't reach the screws, plus a lot are only single pole switching so concerned about frying them.
Assesor seemed happy enough with this, but stressed the fact that ALL cpcs, bonds etc should be in place on the earth bar to maximise the chance of any fault to earth being detected.

 

[Bobby34]

I do split boards half and half, so RCD's off, MCBs on, test from busbars to earth and neutral bars.
Rcbos I test ccts individually with conductors removed from unit, half the time probes can't reach the screws, plus a lot are only single pole switching so concerned about frying them.
Assesor seemed happy enough with this, but stressed the fact that ALL cpcs, bonds etc should be in place on the earth bar to maximise the chance of any fault to earth being detected.

Thanks for the reply, that makes sense, a bit of both, glad you mentioned the assessor was happy thats what I want to make sure really that I don't do something thats out and out wrong

 

[Harry_P]

Tlangdon12 many thanks that is a great video and a handy tip with the croc clip to test L + N to Earth, I think thats the way I will do it for the assessment but still a little confused as I was taught on my course to do the whole board from the main switch with the MCB's and RCD's up and it also shows it like that in Guidance note 3 but I guess it's better to spend an extra couple of minutes rather than risk damaging the breakers.

Thanks again

The way you have been taught to do it tests the whole installation at the same time between live/neutral etc and is called a global ir test. When you test using the live at the top of the mcb you are only testing that individual circuit. So you can choose to test all together or individually. Most choose to test all together (global test) and only test circuits individually if the global ir test gives a low reading which usually means one of the circuits is pulling the reading down, hence why you then test circuits individually to find our which one has the low reading.

 

[Bobby34]

The way you have been taught to do it tests the whole installation at the same time between live/neutral etc and is called a global ir test. When you test using the live at the top of the mcb you are only testing that individual circuit. So you can choose to test all together or individually. Most choose to test all together (global test) and only test circuits individually if the global ir test gives a low reading which usually means one of the circuits is pulling the reading down, hence why you then test circuits individually to find our which one has the low reading.

Thanks, so testing globally doesn't dame the mcbs etc?

 

[Harry_P]

Thanks for the reply, that makes sense, a bit of both, glad you mentioned the assessor was happy thats what I want to make sure really that I don't do something thats out and out wrong

If you get into the habit of following GN3 to the letter you won't do anything wrong. Take the book on site if you can't remember the procedure in GN3 but just get into the habit of doing it exactly as it says

 

[DPG]

If you get into the habit of following GN3 to the letter you won't do anything wrong. Take the book on site if you can't remember the procedure in GN3 but just get into the habit of doing it exactly as it says

But also ensure you understand why you are doing it like that.

 

[Bobby34]

If you get into the habit of following GN3 to the letter you won't do anything wrong. Take the book on site if you can't remember the procedure in GN3 but just get into the habit of doing it exactly as it says

Yea I don't plan to guess anything for a while so anything i'm not 100% sure on I like to try and clarify, like this thread, it seems that either is ok to do but yes il ceetainly be a book worm for a while haha

 

[Harry_P]

Thanks for the reply, that makes sense, a bit of both, glad you mentioned the assessor was happy thats what I want to make sure really that I don't do something thats out and out wrong

If you get into the habit of following GN3 to the letter you won't do anything wrong. Take the book on site if you can't remember the procedure in GN3 but just get into the habit of doing it exactly as it says

Thanks, so testing globally doesn't dame the mcbs etc?

live and neutral connected together tested to earth will make sure you don't damage electronics etc,Also test at 250 v first so if there is a problem you can investigate before you give it the full 500 v

 

[Harry_P]

Yea I don't plan to guess anything for a while so anything i'm not 100% sure on I like to try and clarify, like this thread, it seems that either is ok to do but yes il ceetainly be a book worm for a while haha

I would recommend you build yourself a test unit , get a bit of plywood, get an old consumer unit and wire up at least 2 circuits in it,maybe a socket and a light but do not connect it to a live supply because you will only be doing dead testing.
It will help you to get your head round insulaion resistance testing and especially the difference between global and individual testing,that is the way to get it to sink into your head how it works