Rcd

Hi chaps, wandering if anyone could enlighten me to this, 

Doing a rewire and socket fronts for upstairs power are put on. Power on and press the rcd test button and nothing happens. 

Tell my boss, and he says 'bet you have a N-E fault. When you have N-E fault the rcd won't trip when no loads are connected.' 

So i remove the Neutral in question and the RCD trips on the test button and Sure enough, dead short on N-E on res. I trapped neutral  in a back box. 

Put the circuit under load and the RCD trips. 

So why would having a N-E fault with no load attached stop the RCD test button working???? 

Thanks in advance. 

same reason sa it trips with a load, some power flows to earth, enough to be under the threshold to trip on test. apply a load and some diverts enough to trip it.u

of course, if you had followed your I&T as per BS7671 you would have found the fault on your dead testing and fixed the problem before energising the circuit...

I 'm guessing that there is no third path for the test current to flow  down when pressed  as your trapped neutral has become Earth .  However RCD's usually trip on their own when you crimp the neutral .    Have you tested the Ze ?     

I believe that the test button works by taking current from the sensor inductor input of one pole and feeding it to the output side of  the other pole, so unbalancing the sensor to cause a trip.

The test circuit can be arranged either way, i.e input L to output N, or vice versa.  - Choice of the device designer.

Now, if your device takes its test current from the Live input and feeds it to the Neutral output, AND that neutral is also earthed then the test current will be dumped to earth and the breaker won't trip.

As above, your dead testing should have found this basic fault and power never applied to the circuit until you had corrected the fault.

Did you NOT do any testing prior to switching on and hoping?

just wondering

All dead testing wad carried out, a few sockets were screwed totally back slightly after testing as the plasterer was messing around with plaster around them. The fault occured after they had been in. Be nice to get some knowledge instead of up their own arse dick heads making assumptions.

Cheers tho. 

Have a look at figure 11.0 on page 117 of the on site guide, it shows how a typical RCD test button is arranged. In this example under normal conditions when the test button is pressed some current will flow though the Live winding but not the neutral winding causing an imbalance.  (The value of the test-button winding will be sufficient to produce a trip current). If you then put a dead short between the Earth and the Neutral side of the load, when the test button is pressed some current can also flow through the Neutral winding, which can take the imbalance below the 15ma threshold.

However as has been pointed out this situation should not occur, as leaving live sockets not securely fixed presents a danger to unskilled persons. Either the power should have not been energised, or you should have fixed the socket fronts securely yourself prior to testing & energising. If someone was injured from poking their fingers in the back of your unsecure live sockets I wouldn't like to be trying to explain how I considered it safe practice.

Doc H. 

Just to add, as well as dead testing the circuit, I also test new RCD's / RCBO's first with no outputs at all connected to verify the new RCD/RCBO is actually a good working device. When you have been working for a few more years, you realise that just because something is new out-of-the-box doesn't always mean it is a fully functional device. A useful policy to adopt is, unless you have proved something yourself assume it is faulty.

Doc H.

Newb1 said:

All dead testing wad carried out, a few sockets were screwed totally back slightly after testing as the plasterer was messing around with plaster around them. The fault occured after they had been in. Be nice to get some knowledge instead of up their own arse dick heads making assumptions.

Cheers tho. 

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Common and usual scenario for sites with other trades fiddling about after you have second fixed. Might be worth looking at Yoozybox/Plasterguard/Beadmaster to prevent it on sites like that in future, requires and extra step to come and remove them and screw back but hey no backboxes full of muck/paint all over sockets/faults from gibbons unscrewing things. 

Doc Hudson said:

Just to add, as well as dead testing the circuit, I also test new RCD's / RCBO's first with no outputs at all connected to verify the new RCD/RCBO is actually a good working device. When you have been working for a few more years, you realise that just because something is new out-of-the-box doesn't always mean it is a fully functional device. A useful policy to adopt is, unless you have proved something yourself assume it is faulty.

Doc H.

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Good advice trust nothing these days even IR test cable on the drum before installation since we once has a dodgy batch of LSF T&E that was completely destroyed inside the sheath

Jono Pashley said:

Good advice trust nothing these days even IR test cable on the drum before installation since we once has a dodgy batch of LSF T&E that was completely destroyed inside the sheath

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Absolutely! the exact type of scenario that happens TOO often nowadays.

Doc H.

Newb1 said:

All dead testing wad carried out, a few sockets were screwed totally back slightly after testing as the plasterer was messing around with plaster around them. The fault occured after they had been in. Be nice to get some knowledge instead of up their own arse dick heads making assumptions.

Cheers tho. 

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looks like you dont like the truth

Sorry to say Newb, you can be given knowledge but if you don’t follow the basics then the knowledge is meaningless. 

Whether it’s been tested before or not does not excuse you from carrying out testing of your own prior to energising? 

If anything you gain from this thread then it should be the knowledge to follow the procedures as extracted from bs7671 first. 

Op. You don’t say if it’s a ring or radial, but which ever it is you have go to back to basics and start again with your test and inspection.

out of interest, did you 2nd fix before the plastering was done?

fwiw a n-e fault can stop a red from tripping or make it trip

Murdoch said:

Op. You don’t say if it’s a ring or radial, but which ever it is you have go to back to basics and start again with your test and inspection.

out of interest, did you 2nd fix before the plastering was done?

fwiw a n-e fault can stop a red from tripping or make it trip

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Tell it how it is billy big balls lol.