2391 Question

DoBBY · Jun 5, 2010

Q, List three instrumental tests which are undertaken to confirm the correct operation of a 30mA RCD installed to provide additional protection?

I am not totally sure what answer this question is asking for. Can anybody help.

Is it;-

A. 1/2 x I delta n, 1 x I delta n & 5 x I delta n as you would do on a 30mA RCD

Or is it asking for the actual instrument tests of which I can only think of two e.g The RCD test and Earth fault loop impedence, other than this I can only think of a functional test which is not instrumental.

All replys will be appreciated.

Cheers DoBBY

Scooter · Jun 5, 2010

Hi

I would say that the answer is A, but there is something at the back of my mind that suggests something more. However, just looking through GN3 and C&G 2391 book I can't see anything to suggest anything else.

What I gleen from my swotting is that we sometimes think they are asking for something more complicated, when they are just asking for a straight forward and simple answer such as you have given.

I'm interested to hear from others who may have a different view.

Cheers

pushrod · Jun 6, 2010

There is a ramp test as well but i would say your answer a) is on the right track but i would also include the test currents of 15mA, 30mA and 150mA as from what i have heard from friends presently doing the course they can be really picky on even the smallest things - good luck.

DoBBY · Jun 6, 2010

pushrod said:
There is a ramp test as well but i would say your answer a) is on the right track but i would also include the test currents of 15mA, 30mA and 150mA as from what i have heard from friends presently doing the course they can be really picky on even the smallest things - good luck.

As it says "additional" protection I feel that it only asks for the 30mA tests. Not 100% sure though.

cheekymukka · Jun 6, 2010

DoBBY said:
Q, List three instrumental tests which are undertaken to confirm the correct operation of a 30mA RCD installed to provide additional protection?

I am not totally sure what answer this question is asking for. Can anybody help.

Is it;-

A. 1/2 x I delta n, 1 x I delta n & 5 x I delta n as you would do on a 30mA RCD

Or is it asking for the actual instrument tests of which I can only think of two e.g The RCD test and Earth fault loop impedence, other than this I can only think of a functional test which is not instrumental.

All replys will be appreciated.

Cheers DoBBY

In addition you could do a ramp test if your test kit has the facility, this test in many respects is far more useful than the half times test as what it does is send through the RCD mA's of current slowly ramping it up to a point where the RCD trips. You do this test on both sides of the sinusoidal wave form to obtain the lowest reading this figure will actually establish the mA rating of the RCD your fitted as no 30 mA RCD is ever actually a 30mA RCD - ) guess a bit like a speedo in a car is conservatively manufacturered to prevent a driver actually been over a speed limit.

scotsparky · Jun 6, 2010

As any an RCD it installed to provide additional protection then the answer is to test the RCD so answer A
ramp testing is fine but no space on a BS1671-2008 or NIC test shedule or test result page to record so therefore handy to do but not what is asked for.