Insulation resistance testing live conductors to earth

[An Buachaillín Bán]

Hello all, I hope you're all keeping well.

I have a question regarding insulation resistance testing.

Whether doing a single circuit IR test or a global IR on an entire installation, I usually try to remove all loads, eg lamps, neons etc, but obviously this isn't always possible. So I generally join both L+N and test to earth. This is to avoid damaging any sensitive equipment by ensuring no potential difference across the load.

But how come I still can get a bad reading on a circuit while testing this way? I don't fully understand. Is it due to certain loads being able to throw up a bad IR reading and others not?

Any help would be greatly appreciated

 

[westward10]

What are you considering a bad reading?

 

[An Buachaillín Bán]

What are you considering a bad reading?

Sorry, I probably worded it wrong. Not so much a bad reading, but an indication that there's a load connected somewhere is given a low reading, eg 0.4 or 0.9

 

[westward10]

If this is a global reading you need to break it down to individual circuits by disconnecting the neutral conductors and having all protective devices off. What test voltage are you using.
Having something still connected should not matter unless that something itself is faulty.

 

[Pretty Mouth]

Some older RCD sockets/spurs will give a low IR to E, without apparently being defective. Surge protection (eg extension leads) will give a low IR when tested at 500V (but not at 250), typically about 0.4M ohms. Anything with a functional earth will again give a low IR to E, in my experience this is only likely to be certain RCDs or RCBOs at a downstream CU.

 

[mainline]

Having something still connected should not matter unless that something itself is faulty.

The use of MOVs between L and PE and N and PE is not that uncommon this will cause low IR measurements and not be faulty.

 

[westward10]

What is an MOV?

 

[mainline]

Metal oxide varistors, also known as MOVs are one of the most common types of varistors designed to protect electronic devices and electrical circuits against voltage surges and spikes by changing their resistance.

What is an MOV?

 

[buzzlightyear]

Metal oxide varistors, also known as MOVs are one of the most common types of varistors designed to protect electronic devices and electrical circuits against voltage surges and spikes by changing their resistance.

Wow, you learn some thing every day.
and I thought is was mov, male obesity voice's.

 

[pc1966]

There can be a few reasons for low L+N -> E readings:

• Connected equipment is leaking. By design, or just old/damp
• SPD are connected (but there you get a big difference between 250V where usually > 10M and 500V where usually < 0.1M)
• Cable has degraded or is very damp
• Accessories are damp/dusty

If a global IR is good, as in above a few M, then you know all circuits are similar.

If the global IR gives you cause for concern, say as it is < 1M, then checking individual circuits can at least establish if a low total is due to, say, 10 decent circuits of ~5M leading to ~0.5M, or 9 are > 10M and one is deeply suspect as different at ~0.5M, etc.