Another 2391 question

[La Poste]

Here we go again:

"An electrical installation forms part of a 400/230V three phase four wire TN-S system with an earth fault loop impedance of 0.4Ohms.

A new TP&N distribution board fitted with BS EN 60898 type C circuit breakers has been installed for the new lighting circuits.
The new fluorescent luminaires are surface mounted.
The ground floor luminaires are controlled by a photo cell and the remaining luminaires are controlled solely by circuit breakers.

The installation is carried out in multi core steel wire armoured cables with 2.5mm copper conductors, one of the cores of the cable is used as cpc.

Describe in detail how an insulation resistance test would be carried out on one of the completed ground floor lighting circuits.

The car park is to remain operational while the testing takes place."

I understand how to do an insulation resistance test.
I would also include safe isolation procedures and warning signs
checking of leads and batteries etc.
I would disconnect the photocell and link out.
What I am not sure about is if I can disconnect the lights.
Also because the public is using the car park during the test.
Would it be better to:

1. disconnect all lights, link the disconnected cables together to ensure the circuit remains in tact and test at 500V.
2. Or test the whole circuit at 250V.
3. Or some other way.

The fact they are fluorescent lights and it may be inconvenient to disconnect them and the fact the public are at large is kind of confusing me.

Can anyone offer any assistance with this?
Cheers.

 

[La Poste]

I am thinking the best way to do this is to join line and neutral together and test between them and earth at 500V.
Is 500V allowed under health and safety guidelines with members of the public wandering around? Maybe it would be best to test at 250V.

 

[alanl]

In my view, disconnecting the luminaires and photocells would both give rise to possible re-installation errors,so I'd leave all connected, and join both lines, then test to earth at 250v.
250v as both the photocell and flourescent fittings could be vulnerable to over-voltage, so could be damaged by a 500v test.

I'd be interested in other peoples views on this.

Alan.

 

[telectrix]

agree^^^.also be a good idea to test during the hours of daylight.

 

[IQ Electrical]

The reason for joining L+N together is to prevent voltages across sensitive components so yes, use this method but you still test at 500V dc, this is a City & Guilds question so you stick rigidly to Guidance Note 3 and that doesn't mention reducing test voltages.

 

[alanl]

Of course, yes, you are right IQ Electrical.
Misled by replies I read here, where people say they test at 250v first, 'just in case', there shouldnt be a 'just in case', it should be done properly first time.
Thanks for pointing that out.
Alan.

 

[IQ Electrical]

In the real world, you probably would 'stick a quick 250 down it' just to be sure but C&G aren't interested in the real world...

 

[malcolmsanford]

Why do the GN 3 not recognise reg 612.3.2 part where it says that where it is not reasonably practicable to disconnect sensitive equipment then a voltage of 250v DC can be used.

Just a Sunday morning discussion. I don't have my GN 3 with me at the moment.

 

[IQ Electrical]

I think that regulation note specifically refers to surge protection devices.

 

[malcolmsanford]

"or other equipment that are likely to influence"

So I would interpret that to be honest as anything that would be damaged or influence your test result, and therefore Neons. Lamps, PIR etc.

Would you regard Neons, PIR etc to be covered under 612.3.3 where that reg does not give the 250v value.

Which would make me think in the reg regarding SPD, what is the other equipemnt they refer to ?

As said just a Sunday morning discussion.

 

[IQ Electrical]

I can see how it reads but the reduced voltage test in that note was specifically for SPDs which would be triggered to leak to earth at say 300 Volts.

The note goes on to say "where it is not practicable to disconnect such equipment (eg. fixed socket-outlet incorporating an SPD) the test voltage for the particular circuit may be reduced to 250 V dc."

 

[La Poste]

I wonder about the safety implications of 500V with the public around.
I wonder if I would lose marks by saying I would be concerned about 500V because it is a public place and so would test L+N to earth at 250V quoting health and safety reasons.
Safety first being the most important consideration.

 

[IQ Electrical]

I wonder about the safety implications of 500V with the public around.
I wonder if I would lose marks by saying I would be concerned about 500V because it is a public place and so would test L+N to earth at 250V quoting health and safety reasons.
Safety first being the most important consideration.

You would keep your marks by testing at 500V dc but mitigating the safety concerns, maybe by closing sections of the car park to the public while the testing was being carried out.

 

[La Poste]

There is another question on another paper that involves a motion sensor that cannot be removed.
It's in a toilet and you are asked to do an insulation resistance test on the circuit.
The question asks for two ways to conduct the test.
The first is the same IE linking L+N and testing between them and earth but what about the second.
If we tested at 250V between line and neutral surely we would get funny readings, I am struggling to think of a second method.
Any suggestions?
Thanks.

 

[Chr!s]

Im with Malc on this one.

612.3.2 allows us to reduce the voltage to 250 v d.c, where equipment may be damaged and its not reasonably practicable to remove the equipment.

612.3.3 Also applies, allowing the test to be carried out between live conductors connected together and earth.


2nd paragraph Pg 38 GN3, "Further precautions may also be necessary to avoid damage to some electronic devices". This follows the Test 2 utilizing 612.3.3 and in my opinion leads us to 612.3.2 and the reduced voltage of 250 v d.c.The "Further precaution" being the reduced voltage.

Chris