Earth to live resistance tripping rcd

[animox]

Hi
I have a circuit to a septic tank sump pump, which is tripping the main RCD 30ma

The only fault I can find is a 12K resistance between live and earth, I presume this is more that enough to trip the RCD, but was wondering at what resistance does the rcd trip with this kind of fault?

Or is it slightly more complicated than that?

Cheers

 

[uksparks]

Hi,

You shouldn't have any connection between line and earth.

What tests have you carried out on the circuit? Is it on its own radial direct from the board?

Theres probably a bit of poop on the connector...

 

[uksparks]

actually, where did you measure that resistance? That's on the tank with the circuit disconnected is it? It could be a load if it is and potentially not the problem.

 

[animox]

Its on a radial from its own control box
And that only a multimeter reading between earth and the disconnected live of the radial.

 

[uksparks]

So you have your consumer unit, radial from there, where does that go? Into this control box?

Have you disconnected the pump from the control box? When / if you did did the RCD then hold?

Have you also IR the circuit to the control box? if so, what were the readings... Also whats your R1 + R2 on the circuit?

Is the control box outside?

 

[HandySparks]

Hi
I have a circuit to a septic tank sump pump, which is tripping the main RCD 30ma

The only fault I can find is a 12K resistance between live and earth, I presume this is more that enough to trip the RCD, but was wondering at what resistance does the rcd trip with this kind of fault?

Or is it slightly more complicated than that?

Cheers

12k will give you about 19mA at 230V. A '30mA' RCD should trip between 15 and 30mA. So the low resistance that you've found between L and E is likely to be the cause of the tripping.

 

[brman]

To answer your orginal question. a 30mA RCD will trip if there an imbalance of 30mA between line and neutral currents. That is to say 30mA is flowing from either line or neutral to earth.
In fact the 30mA is the max specification for the RCD, many will trip at around 20 to 25mA (which can be tested with an MFT). So, using ohms law, assuming your supply voltage is around 240V, your 12k resistance from line to earth will give a current flow of 240/12000 = 20mA. Maybe slightly low but close enough that I would not be looking for other faults just yet.

Given you didn't work that out for yourself I am assuming you are not a spark so probably do not have a multifunction or IR tester? I'd be doing what uksparks is suggesting, check each leg of the circuit (CU to control box, control box to pump, pump itself) in isolation (i.e. disconnect the various sections from each other) to find where that 12k is. That will pinpoint the problem - my money is on the pump itself, if not that the control box, particularly if it is outside.

Again, on the assumption you are not a spark, it is worth pointing out a multimeter isn't really the tool for the job but, given you have already spotted the potential problem, it might well be good enough in this instance.hth.....

EDIT: beaten to the important bit by handysparks

 

[NickD]

The other thing to bear in mind is that whatever that 12kohm resistance is, it is unlikely to be a well behaved 12kohm resistance like an actual resistor and more likely to be some non-ohmic random crud or dodgy circuit path through installed equipment. Just because at the 3V (say) that your mutleymeter tests at it passes 0.25mA and therefore measures at 12kohm, it doesn't follow that at about 75 times that voltage (230V) it will pass 75 times the current (19mA) and so still be behaving as a 12kohm resistance.

 

[NickD]

You shouldn't have any connection between line and earth.

Max permitted L-E leakage on the appliance is 3mA if memory serves. Max permitted E-L leakage on the circuit supplying it is bog all given that it should show >2Mohm resistance between the two on a 500V insulation resistance test.

 

[animox]

Thanks guys, got it sorted.
Underground cable was a bit suspect, but the pump had **** itself as well.

£500 all sorted, just hope this one lasts 12 years as well!

 

[NickD]

Thanks guys, got it sorted.
Underground cable was a bit suspect, but the pump had **** itself as well.

£500 all sorted, just hope this one lasts 12 years as well!

Underground cable a bit suspect how? Has it been properly insulation tested at 500V?