main 'Trip' switch opening on change to night rate

[Diver Fred]

I would like some advice please. In the last couple of weeks the 'main' trip switch has operated when the 'dual rate' time switch changes to night rate. Initially this was located down to the cooker circuit but nothing was found to be the cause of the problem. Having checked the whole circuit through including remaking all the terminations the problem did not reoccur until the night storage heaters have been switched on (at the individual wall switches).
One heater is 3KW input, the second 1.5KW input. either heater will cause the trip switch to operate and 'kill' the [power to whole house.

I've metered (but not 'megga'd') the circuits, Live to Neutral; Live to Earth, Neutral to Earth, and see no likely cause. With 3KW heater switched on the meter shows 16 Ohms, 1.5KW shows 33 Ohms.

Thoughts?

 

[bernardgreen]

the 'main' trip switch

Is this an RCD ( with a "test" button ) ?

If it is then there is an earth leakage fault somewhere. It need not be in the storage heaters or the cooker even though the RCD trips when these are turned on.

A small percentage of the large load current of the night storage heaters and / or the cooker on the Neutral can, via the earth fault, bypass the RCD sense coil creating an unbalance between Live current and Neutral current in the RCD and hence the trip.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

 

[John D v2.0]

could be capacitance+resistance between N/E on an existing circuit, causing the extra load of the heaters to only just go over the tripping threshold when the neutral voltage increases relative to earth. It might only happen at the switchovers because things are worse when they're cold (although I struggle to think how as a heater shouldn't have an inrush current).

Not very helpful, but see if it still happens if you turn the heaters on one at a time, and also if you disconnect all other wiring or move to another rcd.

 

[JohnW2]

It might only happen at the switchovers because things are worse when they're cold (although I struggle to think how as a heater shouldn't have an inrush current).

They don't have an 'inrush'current in the same sense as a motor does, but the element will have an appreciably lower resistance/impedance when cold, so more current will be drawn until the element warms up and its resistance/impedance increases.

Whether that is anything to do with the OP's problem is, of course, a much less certain matter!

Kind Regards, John

 

[crystal ball]

Assuming the Main RCD is covering both the off peak and 24 hour consumer units, might be a leaky element(s) after a summer of not being used

 

[EFLImpudence]

could be capacitance+resistance between N/E on an existing circuit, causing the extra load of the heaters to only just go over the tripping threshold when the neutral voltage increases relative to earth.

Would that trip an RCD?

RCDs know nothing about Earth.

 

[JohnW2]

Would that trip an RCD? RCDs know nothing about Earth.

They don't. However, although I don't really think that John D's suggestion is likely to be relevant, an RCD would 'know' about an N-E 'leak' (hence L/N imbalance), whether that leak were via capacitance or anything else. However, given the low N-E pd at the 'worst'of times, I very much doubt that capacitance alone would result in enough N-E current to cause an RCD to trip.

Having said that, I suppose that if, for whatever reason, the L-N imbalance was already 'on the brink', then a very small amount of additional N-E current could be 'the last straw' - although I very much doubt that is what is going on.

Kind Regards, John

 

[EFLImpudence]

Yes, I was more referring (although didn't say) to the "neutral voltage increases relative to earth".

Obviously a (relatively low) resistance to earth will cause the RCD to trip.

 

[JohnD]

John D's suggestion

Nothing to do with me.

 

[John D v2.0]

Sorry my point was indeed that it might be on the edge, hence the suggestion to try with everything else disconnected. If there is a small fault n-e this would only show up when the load is high and therefore n is being pulled away from earth. It could equally well be any other conductor that provides a parallel path back to the substation.
Whether it's capacitance or not is not too relevant, apart from the fact he used a DC multimeter to do the testing. So I was just making sure the op would also consider non resistive paths to earth.

As always, I'm typing on the phone so I tend to be a bit brief in the hopes it's usually better than nothing! But it does sometimes cause a debate!

Hope the op can make head or tail of what's what.

 

[JohnW2]

Yes, I was more referring (although didn't say) to the "neutral voltage increases relative to earth".

Fair enough, but it's obviously that N-E pd which results in an N-E 'leak' through any N-E path (resistive, capacitive or whatever) that may exist.

That pd can be 'appreciable' on a highly loaded circuit. Take, for example, a 'fully-loaded' 32A circuit in a TN-C-S installation with a Ze of about 0.35Ω, and with a 'just OK' Zs of about 1.35Ω, thereby making the R1+R2 of the circuit about 1.00Ω. If we assume 4mm²/1.5mm² cable, that means that R1, hence also Rn, is about 0.27Ω. Hence, with 32A flowing (in L & N), and 'negligible' current flowing in E, the N-E pd at the end of that circuit would be about 8.6V.

That means that in the circuit I have described, it would only require a N-E fault of around 287Ω (or less) to result in a 30mA N-E current that would would cause an RCD to operate. You might well describe that 287Ω as "relatively low", but it is far higher than one could even dream of getting due to capacitance within cabling etc.

Kind Regards, John

 

[JohnW2]

Nothing to do with me.

"JohnD" comes in different 'versions' these days In context, I think it was clear who I meant!

Kind Regards, John

 

[JohnW2]

As always, I'm typing on the phone ....

You have my sympathies!

Kind Regards, John

 

[John D v2.0]

Johnw2, true but firstly the RCD can operate at anything over half that, and depends what else is leaking on the same RCD.

Anyway we can debate this all day but we might as well suggest some things for the op to try and then report back.

 

[EFLImpudence]

... ...
Kind Regards, John

Yes, I know it's all related, but it's the shared current between N and E which causes the RCD to operated; not the pd between them.

After all, L to E has a pd of 240V but has no bearing on the RCD unless some current leaks from it.