RCD Puzzle!

[westie101]

So we're dealing with a curious one at the moment. So I'm interested to hear your views.

We have had reports from a customer (A) that every time one of two neighbours uses their lawn mower, customer A's RCD trips. They are all fed from the same main and are on the same phase.
The system is TN-S (non PME) and the earthing is via the cable sheath with good ELIs.

We keep getting calls to this and despite suggesting that the RCD needs fully tested the electrician seems to be back heeling it to the customer (though he did contact me directly some weeks ago about the problem, to be told to check or change the RCD).
The two houses "causing" this both have RCDs but these are not tripping.
We have confirmed that both mowers are modern and double insulated.

I have dealt with similar where the RCD supplying the sauna in a large house tripped every-time the grass was cut in the adjacent churchyard. We changed the RCD and the problem was resolved!!

My feeling is that the RCD is held closed by an electrical "latch" that is over sensitive so is tripping from the small volt drop caused by the motor start, possibly coupled with a high value of residual leakage in the property. The RCD was fitted when a new kitchen was installed but, as far as we know, the remainder of the wiring was untouched.

 

[Monkeh]

My feeling is that the RCD is held closed by an electrical "latch" that is over sensitive so is tripping from the small volt drop caused by the motor start

It should be mechanically latched, or you'd have to reset it on every power loss.

The installation needs testing fully, including checking leakage current.

 

[JohnW2]

We have had reports from a customer (A) that every time one of two neighbours uses their lawn mower, customer A's RCD trips. ... The two houses "causing" this both have RCDs but these are not tripping. ... We have confirmed that both mowers are modern and double insulated. ... My feeling is that the RCD is held closed by an electrical "latch" that is over sensitive so is tripping from the small volt drop caused by the motor start, possibly coupled with a high value of residual leakage in the property.

How very odd. It's presumably (since it was fitted at the time of a kitchen installation,and therefore presumably not a plug-in active one, or something like that) a passive RCD, so it should not really trip even if the supply disappeared completely, let alone a small voltage drop. In any event, even if that were not the case, I'd probably be intrigued that it was only lawnmowers doing it - I would have thought that most houses contain appliances (e.g washing machines) with greater start-up currents than a lawnmower. Are we sure that it'sonly the lawnmowers that do it?

Kind Regards, John

 

[ericmark]

I have seen some odd ones with RCD's including the 100ma type S tripping every time the 10ma RCD test button was pressed.

In the main after testing and it passing changed it anyway and it's been OK.

We can sit here scratching or heads asking why but in real terms I would swap it first then worry if it continues. I have little trust in the 15ma test as I have had many RCD pass the test but change them and the fault goes.

 

[JohnW2]

I have seen some odd ones with RCD's including the 100ma type S tripping every time the 10ma RCD test button was pressed.

True, but on the face of it, equipment, let alone Class II appliances, used 'next door' should not be able to result in an L/N imbalance of any magnitude - and, as I said, a passive RCD should not care what happens about the supply voltage. However, as you go on to say ...

We can sit here scratching or heads asking why but in real terms I would swap it first then worry if it continues.

.. which I would think is the most sensible course. Indeed, if a new RCD shows the same behaviour, it can be 'swapped back' prior to the 'worrying' (further investigation), so nothing will have been lost.

Kind Regards, John

 

[bernardgreen]

Assuming it is not a cantankerous person who objects to the noise of lawn mowers and is pressing the test button when (s)he hears the mower..........

Does the trip occur when the mower starts ( volt dip ) or when it stops ( voltage transients from the motor.

In all installations there are many stray and parasitic elements. The effect of these when working with a 50 Hz sine wave is insignificant but when working with fast changing transient voltages and higher frequencies these elements do have a significant effect.

In an MCB there is a switch and a complex impedance of resistance and inductance in series with the circuit it is protecting. At 50 Hz the impedance can almost be ignored but at higher frequencies and for transients this impedance will be significant. Common mode transients affecting a circuit ( equal affect on both live and neutral conductors ) will raise or lower the voltage on both conductors by the same amount relative to ground potential. The effect on any equipment on that circuit will be negligable as the voltage difference between live and neutral remains the same. But the effect in the CU will not be negligable. The transient voltages will not be equal when they reach the bus bars in the CU. The transient on the neutral will be not be siginifcantly changed but the transient voltage on the live will be significantly reduced by the inductive impedance in the MCB.

The transient energy on the live that cannot pass through the MCB's impedance will disipate via stray capacity to CPC and neutral and not reach the RCD sense coil. Almost all the transient energy on the neutral will reach the RCD sense coil. It may also be supplimented by the transient energy from the live that was blocked by the MCB's impedance to transients and disipated into the neutral. The RCD will detect a difference in the currents on live and neutral and if large enough trip.

The design of the sense coil in some RCDs suggests they may be much more sensitive to the differences in current when the rate of change of current is faster then the rate of change at 50 Hz

 

[westie101]

We think we may have found part of the problem.

When the kitchen was fitted a new CU was installed which appears to be a split board with an isolator & an RCD.

The original CU was (we think) a MEM unit, somehow the electrician who did the new CU extracted all the contents of the MEM, fitted the main switch into an enclosure and then fed the new CU from the load side!
This "isolator" was faulty and suffered from occasional intermittent contact which would trip the RCD!
A couple of electricians had looked at it and basically walked away because of the state of the installation.

In the end our Fault Technician (who is a sparky) rewired all the tails to do away with the MEM switch, and disconnected 2 unused sw/fuses. Hopefully this will cure it!

 

[RF Lighting]

Sorry for the poor video. I had a similar RCD problem a few years back.



It was a slighly loose neutral on the meter

 

[securespark]

Yeah, I've had that tripping where the tails into a shower RCD were loose, tripped the RCD.

 

[bernardgreen]

The question is " Why do loose connections trip an RCD ? " A loose connection is electrically very similar to a normal switch being turned on and off very rapidly. Turning a switch on or off does not create any unbalance on the live and neutral passing through the sense coil of the RCD.

Or does a normal switch action cause a small un-balance that is well below the trip threshold. And a loose connection creates enough of these small un-balances that their accumulated energy is enough to trip the RCD.

What causes the difference in current between the Live in the sense coil and the Neutral in the sense coil. ?

 

[chivers67]

I've heard a similar story lioke this before somewhere. Can't think where. May have even been on here a couple of years ago. It turned out the next door neighbour had a feed from their house somehow through a party wall.