2391 Insulation Tests at db where rcds and mcbo in circuit

[bob thompson]

Just started with first thread as a new member.

Next week I will be taking the 2391 practical and am in a bit of a quandary. Some of the advice I have seen relating to Insulation Resistance where there is an RCD or RCBO conflicts with my instructors advice. I have not yet confronted the instructor as his was the last advice and was considered as ok: To enable an overall insulation test to be obtained at the db between live conductors and between individual live conductors and earth for the installation it would be sufficient to disconnect the functional earth conductors of any RCBO to remove any influence upon the tests. This suggests that RCBO or RCD could be tested with use of 500V MFT. I am considering erring on the side of caution and undertaking the insulation tests for sub circuits separately where RCDs or RCBOs are in use. I would be grateful for any advice on this subject.

Bob

 

[Apache]

Hello and welcome

I'm sure an actual electrician will be along shortly and answer your question!

 

[Marc]

My tutor's take on this: global IR tests with RCDs in place are the done thing. If there's an RCBO, disconnect the functional earth & either of the live conductors. Spoke to the college technician about it last week, he was of the same opinion.

 

[kme]

To enable an overall insulation test to be obtained at the db between live conductors and between individual live conductors and earth for the installation

it would be sufficient to disconnect the functional earth conductors of any RCBO to remove any influence upon the tests. This suggests that RCBO or RCD could be tested with use of 500V MFT.

I am considering erring on the side of caution and undertaking the insulation tests for sub circuits separately where RCDs or RCBOs are in use. I would be grateful for any advice on this subject.

Bob

Disconnecting the func. earth of RCBo is fine, for live conductors to earth; however you are still subject the electronic components of the residual current device to 2x rated voltage.

I would disconnect one live leg from the component to, as you say, err on caution.

HTH

KME

 

[Des 56]

If the circuit contains voltage sensitive devices,test 2 would be appropriate

Connect line and neutral together and test between this and earth

You can test any circuts that are not vulnerable to damage line to earth and neutral to earth if you wish

 

[ianmacd]

I think the only time I could think of doing a whole site IR test (outside the classroom) would be prior to a CU swap (see Learch, I am using CU now not CCU just to keep you happy ) to check if an RCD will stay on (after uplugging/isolating appliances).

 

[the doctor]

Hi

I do 2391 practicals.....

there is no rcbo, just an rcd main switch on the top single phase consumer unit.....

alan

 

[ADS]

Hi I do 2391 practicals.....

there is no rcbo, just an rcd main switch on the top single phase consumer unit.....

alan

I agree. Exactly the setup in my practical.

Do the ten point IR test on the three phase board with RCD

on single phase board open.

Then, leaving RCD open, test outgoing side of MCBs(open) to earth and

neutral (there's only 3 or 4 circuits), then earth bar to neutral

bar - job done

Don't forget to operate your two way switch to test the other strapper.

Hope that helps.

 

[the doctor]

Just started with first thread as a new member. Next week I will be taking the 2391 practical and am in a bit of a quandary. Some of the advice I have seen relating to Insulation Resistance where there is an RCD or RCBO conflicts with my instructors advice. I have not yet confronted the instructor as his was the last advice and was considered as ok: To enable an overall insulation test to be obtained at the db between live conductors and between individual live conductors and earth for the installation it would be sufficient to disconnect the functional earth conductors of any RCBO to remove any influence upon the tests. This suggests that RCBO or RCD could be tested with use of 500V MFT. I am considering erring on the side of caution and undertaking the insulation tests for sub circuits separately where RCDs or RCBOs are in use. I would be grateful for any advice on this subject.

Bob

Ok Bob i am ready......here is how you do it.

bottom board:

all mcbs off, main switch off(obviously!)

all switch lids on

starter lid on and switched on

top board switched off

test 1(L-E)

crock clip on earth-then test at 500v on .....

all outgoing mcbs

neutral

test 2(L-L)

pop the crock clip on the neutral now then test.....

all outgoing mcbs

on three phase socket circuit test 1-2; 1-3 and 2-3 on outgoing mcb

(remember there could be a short between phases on the armoured!)

test 3 (L-E and L-L on outgoing side of starter)

this is the one they all forget and is a C&G mark point- lift the flap on the 3 phase socket and do all tests "backwards" on the socket holes as there could be a fault on the outgoing side of the starter, though it is remote!)

top board

ok this time...

all lids reassembled

lamps out

switch on 1-w light closed

dimmer removed and switch wire connected to feed (often forgotten)

main switch off

mcbs off

test 1 ( L-E)

pop a crock clip on earth and test

outgoing mcbs (operate 2-w switches to test strappers)

neutral

test 2 (L-L)

pop a crock clip on the neutral and test outgoing mcbs- i would not be bothered about the two ways now- a neutral to strap fault is a no chance)

refit dimmer and carry on!

does this help?

alan

 

[ADS]

Bit more detailed than my description Alan ha ha

One other thing, Bob, if you do it my way the MCBs in the

three phase board are closed - very important

Cheers

 

[the doctor]

quite right ADS,

you can test on the top of the isolator with all the mcbs closed, though it is unorthodoxApplaud Smiley

 

[a1]

Would be interesting to see a few pics of the various college 2391 test rig set ups, as long as it is not against that college's policy.

 

[the doctor]

we shall see....

i cant see any harm myself...

 

[ADS]

quite right ADS,you can test on the top of the isolator with all the mcbs closed, though it is unorthodoxApplaud Smiley

You can't really call it unorthodox - both the OSG (pg83) and GN3 (pg38) tell you to do it this way

Installation as a whole first - break it down to individual circuits if there's a problem.

 

[bob thompson]

Ok Bob i am ready......here is how you do it.bottom board:

all mcbs off, main switch off(obviously!)

all switch lids on

starter lid on and switched on

top board switched off

test 1(L-E)

crock clip on earth-then test at 500v on .....

all outgoing mcbs

neutral

test 2(L-L)

pop the crock clip on the neutral now then test.....

all outgoing mcbs

on three phase socket circuit test 1-2; 1-3 and 2-3 on outgoing mcb

(remember there could be a short between phases on the armoured!)

test 3 (L-E and L-L on outgoing side of starter)

this is the one they all forget and is a C&G mark point- lift the flap on the 3 phase socket and do all tests "backwards" on the socket holes as there could be a fault on the outgoing side of the starter, though it is remote!)

top board

ok this time...

all lids reassembled

lamps out

switch on 1-w light closed

dimmer removed and switch wire connected to feed (often forgotten)

main switch off

mcbs off

test 1 ( L-E)

pop a crock clip on earth and test

outgoing mcbs (operate 2-w switches to test strappers)

neutral

test 2 (L-L)

pop a crock clip on the neutral and test outgoing mcbs- i would not be bothered about the two ways now- a neutral to strap fault is a no chance)

refit dimmer and carry on!

does this help?

alan

Alan

Thanks for this detailed solution which I could have picked up a bit earlier if I had been a bit quicker off the mark. Unfortunately I failed the practical on time expiring before getting into the live checks. I understood the implications of the functional earth for the main RCD in the top board but it caught me out and I lost time and got into a bit of a dispute with the assessor. I will try again and apply a bit of common sense and avoid discussion with assessor in future.

Thanks again.