Installation testing - lighting circuits with lots of electronics

[Tim Watts]

Hi,

One for the electrical chappies :)

I am about to undertake installation and testing of a fairly major pair of domestic lighting circuits - lots of lights, lots of 2 way, lots of electronic parts.

Aside: I *have* done an EAL Level 2 Installation/testing course - basically the 4 day "Part P" entry level qual.

What I was after was *in the real world*:

1) Do most people do the insulation resistance test LN->E (that is L+N bonded together) rather than the more extensive L-E, N-E and L-N tests which would require every bit of electronics to be either bridged (eg dimmer switch) or isolated (eg tube gear, SELV PSUs)

2) Our rather basic course did not cover full testing on the SELV side. Could anyone confirm that SELV wiring should be insulation tested at 250V DC both between conductors (PSU isolated) and 250V betwen mains and SELV side of the PSUs (with inputs and outputs bonded)?

Re (1) - Of course, dimmers should be bridged to conduct a proper R1+R2 test, but I do have a fair few tubes, SELV PSUs and extractor fan controls.

Extensive testing is of course good, but if it becomes too "destructive" (ie the circuits must be broken/bridged for the test, then restored afterwards) then to my mind, it defeats the purpose as a lot of untested disturbances have taken place. A bonded LN->E test at 500V DC seems to make more sense as you are disturbing less and testing is including the insulation in tube gear and (possibly if Class 1) SELV PSUs.

Cheers,

Tim

[ARWadsworth]

Are you sure you want that info?

[Owain]

With judicious use of choc-block you should be able to test L-E, N-E and L-N separately after first fix, before installing the sensitive gubbins. Then you can do a (L+N)-E after second fix.

I would have thought you want an L-N test before going live in case you have an L-N short.

Owain

[John Rumm]

In cases such as yours - yes, that is how I do it.

Generally for the SELV side I just do low ohms continuity end to end, and check the results look sensible and inline with expectations based on the cable CSAs in use.

Indeed. The more destructive testing has more of a place in fault finding IMHO.

[Ben Blaukopf]

Since you haven't started yet - I used these on my recent install of down lights in my hall. Easy to fit through a downlighter hole, might make some things easier for you.

[Tim Watts]

Hehe - I'll re-phrase that as "how would *you* do it?

Because I know you will know how to do it safely but in a practical way :)

As opposed to "how would the the Barratts sparky do it?" (ie do one, if lucky, photocopy 30 times and write individual site addresses on each copy!).

[Tim Watts]

This is a good point. One could always do that test at 250V DC with all parts connected.

[Tim Watts]

Thanks John - that is very helpful :)

At least the smoke detectors (part of this circuit) are easy - all mine unplug off the base - except for the master silencing switch on the wall.

Cheers,

Tim

[Tim Watts]

[ARWadsworth]

I'll giver you proper advice later.

But where did you get the info about the 30 photocopys from?

[Brian Gaff]

You certainly have the right surname for the job though.

Brian

[ARWadsworth]

I was going to scan the relevant bits from my book, but I now have to go to work (unexpectedly)

1) Do most people do the insulation resistance test LN->E (that is L+N bonded together) rather than the more extensive L-E, N-E and L-N tests which would require every bit of electronics to be either bridged (eg dimmer switch) or isolated (eg tube gear, SELV PSUs)

Yes. That is what I do

2) Our rather basic course did not cover full testing on the SELV side. Could anyone confirm that SELV wiring should be insulation tested at 250V DC both between conductors (PSU isolated) and 250V betwen mains and SELV side of the PSUs (with inputs and outputs bonded)? 250V on the SELV side and 500V betwen the mains and SELV.

Personally I would not bother testing that bit.

[Tim Watts]

Ta :)

Why - is it true? I made it up - though the IET forums occasionally allude to "extrapolated statistical testing procedures" by less reputable persons on cheap house bashing jobs ;-o

[Tim Watts]

Thanks Adam - I was keen to see what practical measures were deemed acceptable in practise.

OK.

I could not see a huge amount of point in IR testing the SELV cabling - especially if it a short bit. Continuity is probably useful though (fire risk especially if junction boxes are used).

Many many thanks,

Tim

[Andy Wade]

Otherwise known as the 'drive-by PIR'.

[ARWadsworth]

[Tim Watts]

Hi Adam,

OK - I see their point... I've kept my SELV physically seperated from mains (either clipped to seperate (or seperate sides of) rafters or in seperate adjacent wire trays. Now, I have bonded my trays to main earth[1] - so I suppose a 250V test of the SELV to tray would make sense... The possibility of it becoming connected to mains even by total catastrophic insulation failure is more or less zero, but it could conceivably cease to be SELV by contacting an earthed part.

[1] a debateable point. Not an extraneous conductor, but as it is full of cables which may become connected to the tray in the event of overheating, fire or mice, it made sense...

Cheers,

Tim