PME Trip Times etc advice

[EFLImpudence]

I find it a little hard to see how having loads connected to the circuit could increase measured RCD trip times.

In one of my replies I stated that nothing could increase the tripping time.
No one has contradicted.

However, and I'm just theorising as I don't know exactly how the meter works.
What if, with the loads connected, there was a, say 10mA, leakage from L to E and the meter induces a leakage current from N to E, the first 10mA of which would counteract the existing fault.

Presumably this would increase the tripping time.
A ramp test would give a really high value too.


However if the existing leakage were from N to E this would reduce the tripping time but the fault may be discovered because the IΔn/2 test could also cause a trip (the existing fault may have to be slightly higher).
The ramp test value being correspondingly lower.

 

[JohnW2]

I find it a little hard to see how having loads connected to the circuit could increase measured RCD trip times.

In one of my replies I stated that nothing could increase the tripping time. No one has contradicted.

Yes, it's literally true that no-one has cointradicted you. However, it is being suggested that the x5 trip times measurements (previously high)should be repeated with no load, the implication presumably being that the previous high results could have been erroneous results due to the circuit having loads connected.

However, and I'm just theorising as I don't know exactly how the meter works. What if, with the loads connected, there was a, say 10mA, leakage from L to E and the meter induces a leakage current from N to E, the first 10mA of which would counteract the existing fault. Presumably this would increase the tripping time.

I don't think that meters can possibly work that way around - how could they induce a known N-E leakage current? They could connect N and E together (directly or through a resistance) but that would generally result in no current if there were no loads on the RCD, and an unpredictable current if there were loads. The meters surely must introduce a known L-E current, by connecting a known resistance between L and E?

However, what you say does introduce one possibility. Assuming (as above) that the meter introduces a L-E current, if there is an existing N-E leakage due to a load, then I agree that the 'first bit' of the meter's induced test current would be 'cancelling' that pre-existing N-E leakage, hence decreasing the overall L-N imbalance and hence possibly resulting in increased tripping times. I hadn't thought of that possibility.

Mind you, that effect could not be all that great when measuring at x5. Any pre-existing N-E leakage would have to be below 30mA (quite a lot lower in practice, since '30mA' RCDs usually trip at <30mA), otherwise the RCD would already have operated, so the very worst case would be to reduce the trip current from x5 to x4 - in practice probably a lot less of a difference than that.

Kind Regards, John

 

[EFLImpudence]

However, it is being suggested that the x5 trip times measurements (previously high)should be repeated with no load, the implication presumably being that the previous high results could have been erroneous results due to the circuit having loads connected.

My last post was in answer to why RCDs should be tested with no loads connected rather than this thread.
Whilst the limit for 1xI&#916;n tests is 300ms it is very rare for the result to be over 40ms (the 5xI&#916;n allowed maximum) so, if that is the case, does it follow that the 5x test is superfluous and doesn't really need to be done?

I don't think that meters can possibly work that way around - how could they induce a known N-E leakage current? They could connect N and E together (directly or through a resistance) but that would generally result in no current if there were no loads on the RCD, and an unpredictable current if there were loads. The meters surely must introduce a known L-E current, by connecting a known resistance between L and E?

Possibly so.
I don't know how 'clever' the meters are.
Is that why the 0 and 180 results are different.

However, what you say does introduce one possibility. Assuming (as above) that the meter introduces a L-E current, if there is an existing N-E leakage due to a load, then I agree that the 'first bit' of the meter's induced test current would be 'cancelling' that pre-existing N-E leakage, hence decreasing the overall L-N imbalance and hence possibly resulting in increased tripping times. I hadn't thought of that possibility.

Mind you, that effect could not be all that great when measuring at x5. Any pre-existing N-E leakage would have to be below 30mA (quite a lot lower in practice, since '30mA' RCDs usually trip at <30mA), otherwise the RCD would already have operated, so the very worst case would be to reduce the trip current from x5 to x4 - in practice probably a lot less of a difference than that.

Quite but then it would not trip on the 1xI&#916;n test and be noticed.

 

[JohnW2]

Whilst the limit for 1xI&#916;n tests is 300ms it is very rare for the result to be over 40ms (the 5xI&#916;n allowed maximum) so, if that is the case, does it follow that the 5x test is superfluous and doesn't really need to be done?

Yes, I think that follows - if trip time at 1 x I&#916;n is not over 40mA, then testing at 5 x I&#916;n is essentially redundant. However, we don't know that such was the situation in the OP's case - all we know is that the trip times at 1 x I&#916;n were all <300ms.

Possibly so. I don't know how 'clever' the meters are.

I don't think it matters how clever the meter is - I don't think there is any way a meter could introduce a known N-E current ,for the reasons I described. I feel sure that it has to be an L-E current that it induces for the test.

Is that why the 0 and 180 results are different.

I don't think so - that is a different issue.

Quite but then it would not trip on the 1xI&#916;n test and be noticed.

That depends upon what current would have tripped the RCD in the absence of an N-E leak/fault. If, say, 20mA would have tripped it, then any N-E current up to 10mA will not prevent it tripping at 1 x I&#916;n (i.e.30mA).

Kind Regards, John

 

[EFLImpudence]

Quite but then it would not trip on the 1xI&#916;n test and be noticed.

That depends upon what current would have tripped the RCD in the absence of an N-E leak/fault. If, say, 20mA would have tripped it, then any N-E current up to 10mA will not prevent it tripping at 1 x I&#916;n (i.e.30mA)

Obviously 'it depends' but -

An RCD which trips at 25mA.
An N to E fault of 11mA on the installation.
A tester applied L to E fault of 30mA - RCD will not trip.
Ramp test will show 36mA.

Or.
An RCD which trips at 25mA.
An L to E fault of 11mA on the installation.
A tester applied L to E fault of 15mA - RCD will trip.
Ramp test will show 14mA.

If that's how the meters work.

 

[JohnW2]

deleted

 

[JohnW2]

Obviously 'it depends' but - An RCD which trips at 25mA. An N to E fault of 11mA on the installation. A tester applied L to E fault of 30mA - RCD will not trip. Ramp test will show 36mA.

Agreed, I would assume that's how the meters work.

... Or. ... An RCD which trips at 25mA. An L to E fault of 11mA on the installation. A tester applied L to E fault of 15mA - RCD will trip.

Did you mean 14mA, rather than 15mA (11+14=25)? However, I didn't actually think that was how the meters worked when measuring trip times (as opposed to doing ramp tests). I thought (but may be wrong) that they simply applied the appropriate test current (30mA in this case) 'from the start' - rather than gradually increasing it until a trip occurs (per ramp testing), as you seem to be suggesting. After all, you are meant to be doing the test AT 1 x I&#916;n, not at whatever current causes a trip.

Ramp test will show 14mA.
If that's how the meters work.

Yep, that's my understanding of how the meters work. What is your point - that the remp test will reveal the presence of the installation leak?

Kind Regards, John

 

[EFLImpudence]

Did you mean 14mA, rather than 15mA (11+14=25)?

No. 15mA is the I&#916;n/2 test where the device shout NOT trip but with a fault of 11mA already there making 26mA the RCD WILL trip.

However, I didn't actually think that was how the meters worked when measuring trip times (as opposed to doing ramp tests). I thought (but may be wrong) that they simply applied the appropriate test current (30mA in this case) 'from the start' - rather than gradually increasing it until a trip occurs (per ramp testing), as you seem to be suggesting. After all, you are meant to be doing the test AT 1 x I&#916;n, not at whatever current causes a trip.

Already explained?

Ramp test will show 14mA.
If that's how the meters work.

Yep, that's my understanding of how the meters work. What is your point - that the remp test will reveal the presence of the installation leak?

My point is, as said, to do with the effects of testing RCDs with loads attached if a fault or faults are present.

So if this is not realised and only a small fault were present the results would be invalid.

Also, yes, the ramp test is is a method of finding on which circuit a fault is - by disconnecting one, some or all the circuits you can see which one has the fault.

We have earth leakage meters now - if you can get at the conductors.

 

[JohnW2]

Did you mean 14mA, rather than 15mA (11+14=25)?

No. 15mA is the I&#916;n/2 test where the device shout NOT trip but with a fault of 11mA already there making 26mA the RCD WILL trip.

Oh, fair enough - I totally misunderstood you. As you presumably realise, I thought that you were talking about a 1xI&#916;n test, and were suggesting that the test current would be gradually increased, up to a maximum of 1xI&#916;n, until the RCD tripped (i.e. like a ramp test).

My point is, as said, to do with the effects of testing RCDs with loads attached if a fault or faults are present. So if this is not realised and only a small fault were present the results would be invalid.

I've never had any disagreement with that in relation to trip currents (and trip tests). My comments were all about trip times (per this thread), and I thought that, although faults on the installation (or loads) could decrease trip times, I couldn't see how they could ever increase trip times. However, that was because I was only thinking about L-E leaks and you rightly made me also consider the possibility of N-E leaks - which I agree could increase the measured trip times.

Kind Regards, John

 

[ricicle]

A N-E leakage on a circuit will cause premature tripping of the RCD according to the test meter, the same as a L-E fault, because the leakage current is only going through the L coil on the toroid.

Draw it out !

 

[JohnW2]

A N-E leakage on a circuit will cause premature tripping of the RCD according to the test meter, the same as a L-E fault, because the leakage current is only going through the L coil on the toroid. Draw it out !

Ah - so are yoiu suggesting that I was right in the first place ?! Watch this space - I'll be back after I've done some 'drawing out'!

Kind Regards, John

 

[JohnW2]

A N-E leakage on a circuit will cause premature tripping of the RCD according to the test meter, the same as a L-E fault, because the leakage current is only going through the L coil on the toroid. Draw it out !

Ah - so are yoiu suggesting that I was right in the first place ?! Watch this space - I'll be back after I've done some 'drawing out'!

Back! Yep, ricicle, I cannot disagree with you. EFLI, what have you got to say for yourself for having seemingly led me astray?

Kind Regards, John

 

[JohnW2]

Back! Yep, ricicle, I cannot disagree with you. EFLI, what have you got to say for yourself for having seemingly led me astray?

I should perhaps have added that even if people start hypothesising about funny things going on on the supply side of the RCD, resulting in a current going 'backwards' through the N coil of the RCD (and a N-E fault in the RCD protected circuit), that will still probably be additive with other L-N imbalances since, although going through the N coil, it's phase is reversed.

Kind Regards, John

 

[EFLImpudence]

EFLI, what have you got to say for yourself for having seemingly led me astray?

Well, if I can't rely on you to put me right straight away, is it worth carrying on?

Sorry. Thanks ricicle.

 

[JohnW2]

Well, if I can't rely on you to put me right straight away, is it worth carrying on? Sorry. Thanks ricicle.

Seriously - even if it steered me away from my original intuition (about trip times) your mistake was one of those which would be so easy for any of us to make - and it was one of those situations in which, when I (and I'm sure others) read quickly what you had written, it 'felt' so totally reasonable that it did not occur to be to think more deeply or challenge it!

Kind Regards, John