Testing PFC on a three phase board with 100 amp From the meters tails

[haydengraham24]

I’m currently doing some testing on some site temps I have a three phase trip system the tails from the head go into a 100 amp RCD which feeds a board I’m trying to do a pfc on the db but it comes up with 2 lead high which trips the RCD I tried on 2 lead low however came up with 20 amps which didn’t seem right what’s the best way to get round this

 

[davesparks]

What do you mean by "it only comes up with 2 lead high"?

 

[haydengraham24]

What do you mean by "it only comes up with 2 lead high"?

When I set it to L - PE on the settings it comes up with 2 Lead High but this would trip the RCD it did have a 2 lead low but the reading didn’t seem right when I tried this setting only just got this tester so trying to figure it out

 

[pc1966]

Can't you get to the RCD input terminals with your probe?

The low current / non-trip results are often not that accurate, but 20A is very suspiciously low even for that.

 

[haydengraham24]

Can't you get to the RCD input terminals with your probe?

The low current / non-trip results are often not that accurate, but 20A is very suspiciously low even for that.

I could try and get to the RCD Input terminals just wanted to check it in the atchual DB aswell would it make to much difference taking the reading at the RCD ?

 

[SparkySy]

If you can access the RCD input terminals then you can conduct a high current loop test. Some RCD's can cause uplift which skews the results of your test.
That does seem low is this a TT supply by any chance?
If so what are your Ra / Ze readings? Quite high I would guess!

If you can get an accurate measurement at the RCD then you could take an R2 or R1+R2 from the next unit along and calculate the PFC/ Zdb from that.

 

[pc1966]

I could try and get to the RCD Input terminals just wanted to check it in the atchual DB aswell would it make to much difference taking the reading at the RCD ?

In the ideal world the RCD would make no difference as it should be only milliohm contact resistance at most, so your result should be dominated by the supply characteristics.

However, RCD have a sense coil designed to pick up the Earth fault current and that can lead to "RCD uplift" where the measurement of PFC is fooled by the differential sense transformer leading to an unrealistically low PFC / high Ze/Zs result. I thought most modern MFT or similar had tricks to reduce this effect but a check on the RCD input should show you a realistic value.

 

[SparkySy]

My Megger 1741 is supposed to have that ability, but recently whilst testing some old BG type AC RCDs found that it doesn't always work as it should!

 

[davesparks]

When I set it to L - PE on the settings it comes up with 2 Lead High but this would trip the RCD it did have a 2 lead low but the reading didn’t seem right when I tried this setting only just got this tester so trying to figure it out

2 lead low current test should be a last resort as they are notoriously bad for uplift. Use a 3 wire low current test for a better chance of getting the right result.

You should be using the tester to carry out the tests you want, not letting the tester dictate what you are doing.