Understanding what this means on the megger 1731

[HappyHippyDad]

I am in the process of hunting for another difficult fault (intermittent tripping RCD, absolutely no faults showing on any circuits and only 2mA fault current in Main earth).

My question does indirectly relate to this... perhaps.

When testing the RCD with my megger 1731 I am not getting a reading in mS. All I get is 'trp'. The RCD trips, but it does it so quickly and just states 'trp'.

The above was testing at a socket so I tried testing at the RCD itself and got the same.

The RCD is protecting 3 circuits, 2 socket circuits and the cooker. When i test at the RCD with the 3 breakers up the RCD trips and says 'trp'. When I switch off the cooker breaker and test I then get a proper reading (27mS). When I switch the sockets off and the cooker on I get a proper reading also (27mS). What does this mean?

Reading through the megger download does not give any useful info. It just says (under RCD testing), "if the RCD trips it will show 'trp'!!!!! (see pic below)

 

[westward10]

It's tripping before the tester goes into its test sequence for ×1.

 

[Lucien Nunes]

I was assuming it was an 'auto' test where it does 0.5x, 1x & 5x in sequence, which is how I usually use mine. I think this will display TRP if it trips on 0.5x. If you were on the fixed 1x range then I expect the tester produces a small amount of 'leakage' while synchronising its test current injection circuit to the mains waveform, and even that is enough to push your RCD over the limit.

 

[davesparks]

only 2mA fault current in Main earth).

When i test at the RCD with the 3 breakers up the RCD trips and says 'trp'. When I switch off the cooker breaker and test I then get a proper reading (27mS). When I switch the sockets off and the cooker on I get a proper reading also (27mS). What does this mean?

A measured current in the earthing conductor means very little, this kind of current can be measured on an installation with no faults at all.
To measure earth leakage you need to measure the imbalance between current in all live conductors, this is done by clamping around all live conductors at once.

Those test results suggest that the RCD is fine, but that there is earth leakage on both of those circuits which is biasing the RCD to trip at a lower current than it should.

 

[HappyHippyDad]

A measured current in the earthing conductor means very little, this kind of current can be measured on an installation with no faults at all.
To measure earth leakage you need to measure the imbalance between current in all live conductors, this is done by clamping around all live conductors at once.

Those test results suggest that the RCD is fine, but that there is earth leakage on both of those circuits which is biasing the RCD to trip at a lower current than it should.

Thanks for this informative reply.
When you say clamp around all live conductors at once do you mean both the supply live and neutral? If so, why will this reading be different to clamping the main earth and bonding conductors?
Any difference shown when clamping the supply live and neutral would suggest earth leakage. Doesn't this current leak through either the main earth or the bonding conductors?

 

[Bobster]

Thanks for this informative reply.
When you say clamp around all live conductors at once do you mean both the supply live and neutral? If so, why will this reading be different to clamping the main earth and bonding conductors?
Any difference shown when clamping the supply live and neutral would suggest earth leakage. Doesn't this current leak through either the main earth or the bonding conductors?

Yep, round both incoming Line and neutral. This wil show the imbalance, in other words leakage.

In theory clamping all bonding conductors and main earth should be the same thing, however this will never actually be the case. Some current can stray elsewhere, through pipes past the bonding conductor into the actual ground, moisture to ground etc...

 

[davesparks]

Doesn't this current leak through either the main earth or the bonding conductors?

In an ideal world it would yes, but in reality some will flow elsewhere too. Parallel paths, fortuitous connections to earth or even the fault you are trying to find could be diverting the leakage current elsewhere.

 

[Bobster]

In an ideal world it would yes, but in reality some will flow elsewhere too. Parallel paths, fortuitous connections to earth or even the fault you are trying to find could be diverting the leakage current elsewhere.

I imagine the easiest way of imagining it is a bonding conductor connected to a pipe for example, is just a potential divider.

 

[Strima]

When you say clamp around all live conductors at once do you mean both the supply live and neutral? If so, why will this reading be different to clamping the main earth and bonding conductors?

Both live conductors will have magnetic influence on each other due to current draw but these are cancelled out by each other. If there's some leakage going a different route there will be a part of the magnetic field that isn't cancelled out giving you a reading.

I tend to test with all conductors clamped, both lives and earth, then just the live conductors. This gives a ball park figure of what is finding parallel paths and what is going back down the main earth conductor.

 

[Midwest]

Thought the best method for earth leakage testing, was clamping around live/neutral supply?

Edit beaten to it.

 

[AJshep]

I would clamp around the L-N of the outgoing problem RCD.
I tried to say this earlier in the thread but after re-reading my previous doesn't make much sense.