Testing RCD

[hood2]

Hi all

I have been on a DISQ course recently on which they said to get the reading of Zs on radial circuits instead of probing onto live conductors use the formula Zs = Ze + R1 + R2 as they don't like the potential danger. Where as with a ring final circuit you can use the plug eliminating any potential danger.SO on the RCD testing section do you only test circuits that you can plug into or use probes to do your RCD test on every circuit, which in my eyes would be better as each circuit would have different characteristics which would effect tripping times, but this is again potentialy dangerous as you are working on live terminals possibly on ladders.
sorry if this sounds like a stupid question but i cannot find a answer in any of the books i have. Any input it much appreciated.

 

[Amp David]

You can still use your plug top lead on a socket radial.

 

[hood2]

what formula could i use to find out thetripping times of a 30ma rcd at x1 and x5. thanks

 

[roukel01]

strictly speaking, you should test the rcd with a loads disconnected and at the source (i.e at the actual RCD) however, testing at a nearby socket is acceptable.
If one RCD is protecting a few circuits, you dont need to test it on each circuit. Earth continuity tests and Zs tests confirm that an earth is present throughout each circuit, and so providing you have tested and proved the RCD works at its source, you don't need to carry on and test it anywhere else.

 

[hood2]

I have just installed a new CU therefore split the circuits accordingly and ended up with 2 ring final circuits on 1 RCD and 2 on the other RCD when doing the auto RCD test on each circuit each one gives a slightly different reading. Thats why was wondering if needed to do the RCD test for each circuit on each side in case any circuit didn't disconnect in time

 

[roukel01]

no, because the RCD requires a maximum resistance in a circuit of 1667 ohms to operate at 30mA and 50V touch voltage, and so, if your Zs readings are ok and you've tested the RCD disconnected from any load, you know it should work correctly and in the required time on any circuit it is connected to

 

[ASJayes]

If you do multiple tests on an single RCD you will get results that differ slightly between each reading. RCD warms up and temperature affects tripping charactoristics.

 

[roukel01]

If you do multiple tests on an single RCD you will get results that differ slightly between each reading. RCD warms up and temperature affects tripping charactoristics.

You should take your first reading.... Unfortunatley an RCD doesn't have chance to "warm up" when a real fault occurs, hence why you should test with loads disconnected etc etc to give it a cold test

 

[Davey101]

Is it just me, or is it slightly ridiculous to be worrying about testing live circuits? (this is directed at the course designers/lecturers NOT at the op).
How the hell do you test the characteristics of a D/board,or do a dead test on a circuit in a live board? Dont tell me they said you should turn off the Dis board or consumer unit before taking the cover off?!
RCD readings will often differ SLIGHTLY, like others have said, its easier to test them at the DB.
Although you can calculate Zs, I think an earth loop test is important, as it proves that the circuit is safe when you have finished testing. Who's to say that you have connected the circuit, especially the cpc correctly, after you have finished your dead tests? The loop test pretty much proves this, as it is (or should be) the last test you do, after you have screwed everything back together.
Just my 2p's.

 

[Sintra]

Is it just me, or is it slightly ridiculous to be worrying about testing live circuits? (this is directed at the course designers/lecturers NOT at the op).
How the hell do you test the characteristics of a D/board,or do a dead test on a circuit in a live board? Dont tell me they said you should turn off the Dis board or consumer unit before taking the cover off?!

Agree with you Davey totally impractible to isolate complete DB especially when doing PIR's on occupied premises.

 

[Spudnik]

Although you can calculate Zs, I think an earth loop test is important, as it proves that the circuit is safe when you have finished testing. Who's to say that you have connected the circuit, especially the cpc correctly, after you have finished your dead tests? The loop test pretty much proves this, as it is (or should be) the last test you do, after you have screwed everything back together.
Just my 2p's.

Your R1+R2 test will confirm everything is connected correctly before the installation is energised.

Zs test is almost a confirmation really.

 

[Davey101]

Your R1+R2 test will confirm everything is connected correctly before the installation is energised.

Zs test is almost a confirmation really.

What cables do you disconnect to carry out an R1+R2 test? (or r1. r2, rn or IR for that matter?)

 

[Mr Mark Sparks]

What cables do you disconnect to carry out an R1+R2 test? (or r1. r2, rn or IR for that matter?)

thats going to need to longer answer for me

treat yourself to one of these it all you need to know

http://www.amazon.co.uk/dp/0863418570/?tag=googhydr-21&hvadid=4765375141&ref=pd_sl_2e7pmz4use_e


btw i agree with jason the r1 r2 is a far more accurate test

 

[Davey101]

lol, I wasn't asking because I didn't know, just trying to prove a point about the need for a loop test.
Its hard to get your point across using a keyboard sometimes

I guess its just what you are used to, with most sparks I know, you are very unlikely not to carry out a loop test, but may not do an R1+R2 (I'm not saying thats right btw).

 

[Spudnik]

Sorry, meant to say r1 + r2.