Reason for testing Live Neutral to Earth

[Alan Marriott]

Hi guys

Just wanted some help in finding out the exact reason in why we test Live Neutral to Earth in lighting circuits.

I understand it in case any loads are still connected and to not damage sensitive equipment but why does it damage it?

Would testing Live- Neutral on 230v still damage it?

Just looking for the technical/real explainantion please guys!

Many thanks

 

[Marvo]

I assume you means IR testing.

You can test using 250 volts between Live and Neutral without causing damage but it's better to use a test voltage that's higher than the supply voltage ie 500v and this can damage appliances if you test L-N.

 

[davesparks]

Why are you testing lighting circuits this way? If you are doing initial verification you should be testing L-N L-E and N-E

For other testing there is no reason to test all lighting circuits with L and N linked, not any reason to restrict it to just lighting circuits if necessary.

 

[ipf]

I presume we're talking EICR here.
250v testing wouldn't cause problems....but it's not testing at 500v, as required. Besides, if you still had results problems between L+N with 250, what would be the point?
Could be any circuit, anyway.

 

[dmxtothemax]

quote - [ but why does it damage it? ]
Depends on the test voltage,
Usually with megger style ,it is tested at twice normal running voltage, so a 240v circuit would be tested at 500v.
So 500v could damage something designed for 230 / 240v.

 

[Alan Marriott]

Hi

Thanks for the replies. Yeah I know that with new installation you should always test L-N,L—E,N-E and also know that you should ideally test at 500v to make sure everything’s clear.

My question is still though what is the reasoning behind L+N to E at 230v?

Why does testing between them conductors at 250v not damage any equipment and a test of L- N would?

I’m not being awkward just curious to know the reasoning behind it.

Many thanks again guys

 

[Pete999]

Hi

Thanks for the replies. Yeah I know that with new installation you should always test L-N,L—E,N-E and also know that you should ideally test at 500v to make sure everything’s clear.

My question is still though what is the reasoning behind L+N to E at 230v?

Why does testing between them conductors at 250v not damage any equipment and a test of L- N would?

I’m not being awkward just curious to know the reasoning behind it.

Many thanks again guys

The previous post to this one has answered your question.

 

[Cadgey123]

There is no potential between L-N if they are joined together so you can’t damage what’s connected between L-N

 

[static zap]

Think cheap dimmer parts , maybe suppressing interference .
A higher voltage rating costs more , so they get made with cheaper
-Only just good enough parts-
..Something similar occurs in RFC with USB "Sockets".
( Its always the more pricey + fancy electronic bits that die )

 

[Deleted member 9648]

It is often simply not practical to test L-N on an existing circuit, imagine a kitchen with 20 downlights, you'd have to go round and remove lamps, disconnect drivers/transformers, damage to ceiling etc, because you'll get a reading through connected loads regardless of test voltage, therefore the test will not prove anything. The L/N-E test is as much because of practicalities as the risk of damage .

 

[Marvo]

........My question is still though what is the reasoning behind L+N to E at 230v?

Why does testing between them conductors at 250v not damage any equipment and a test of L- N would?.

In an appliance the control board is supplied by the live and the neutral and it's designed to receive 240 volts. If you IR test between L-N at 250 volts it won't damage it. If however you do the test properly at 500v the control PCB receives double the voltage it's designed to and there's a possibility it gets damaged.

If you connect the L+N together and IR test between them and earth then the control board receives zero volts because the wires supplying it are connected, hence it won't be damaged by the test.

Low insulation to earth is the most important IR test because it's user safety related, on an existing circuit that's already in use there's rarely a need to IR test between the line conductors (L-N). On a circuit in a new installation you need to do the full IR test including L-N but there's not likely anything plugged in so no risk of damage.

 

[Wilko]

Noting all the good words so far - I would add that connected loads are designed for 230V RMS (plus a bit of course) so that's 400V peaks. The 250V DC test is below this and the 500V DC test above it. Testing L to N with connected loads won't achieve much, as the loads are likely to be 20 Ohms to 20 kOhms say.