Motor Tripping RCD but it's not even connected to it

JD Hogg · Oct 5, 2017

Hi, I have just installed a dedicated 32A circuit to feed a motor which according to the data plate is 3.0kW and 16.6 Amps, single phase. It is fed from a 32A MCB via 6mm cable about 22 metres. I have fed it from a MK twin 63 Amp RCD board but this motor circuit is not protected by either RCD,

The confusing thing is that one of the RCD's trips when the motor is started; when the motor is running I can reset the RCD and everything is fine. The RCD that trips is the one physically located nearest to the motor circuit RCD

There are no borrowed neutrals, I have swapped the RCD's over to make sure one of them isn't faulty but the RCD nearest the motor circuit still trips when the motor is started.

Obviously there is going to be a voltage dip on start up but would this trip an RCD and if so why don't they both trip?

There is no soft start on the motor.

Any ideas appreciated.

Cadgey123 · Oct 5, 2017

westward10 said:
This is being caused by the motor starting, rcds and motors often don't make good friends.

I agree, sounds like the case. As long as it’s wired up right

Pete999 · Oct 5, 2017

A question that hasn't been asked yet, is the CU a high integrity board? just so I can get my head around the problem.

westward10 · Oct 5, 2017

Pete999 said:
A question that hasn't been asked yet, is the CU a high integrity board? just so I can get my head around the problem.

It is a TT system needs rcd for fault protection.

streamer · Oct 5, 2017

JD Hogg said:
32A MCB feeding motor, 16A MCB feeding different motor both 32A and 16A MCB's fed directly from main switch

@Pete999 This quote is part of post #10, so it looks like it's a high integrity board. The confusion is that the motor is not connected to the RCD which is tripping.

@JD Hogg If it is a TT system how can you have circuits not connected to an RCD?

Billsays · Oct 5, 2017

Is it possible to move the problem circuit to between the Main switch and the 1st RCD? might stop the heavy load affecting the RCDs. Or possibly fit another small non RCD ccu just for that circuit?

JD Hogg · Oct 5, 2017

@JD Hogg If it is a TT system how can you have circuits not connected to an RCD?[/QUOTE]

There is a 100mA RCD before this board but this isn't affected.

Wilko · Oct 5, 2017

Just trying to understand the set up - has this board itself got its own means of earthing, or is it derived from the main board ?

Pete999 · Oct 5, 2017

westward10 said:
It is a TT system needs rcd for fault protection.

Yes I appreciate that Westy, but the OP has mentioned in his post that the cb for the motor is fed from the main switch, or I think that's what he means#10.
Streamer, the reason I asked about the high integrity board was the OP mentions the Motor cb is fed from the MS, I was asking mainly because the OP may have adapted the board to suit his needs, it has been known to happen .
If he has then the borrowed neutral theory does hold up despite the OPs assurance that there is not a borrowed N, OP I'm not having a dig, just can't get my head around the problem.

Pete999 · Oct 5, 2017

Billsays said:
Is it possible to move the problem circuit to between the Main switch and the 1st RCD? might stop the heavy load affecting the RCDs. Or possibly fit another small non RCD ccu just for that circuit?

Loading should not trip an RCD.

Billsays · Oct 5, 2017

Pete999 said:
Loading should not trip an RCD.

True but RCDs can be twitchy things when motors are around. It was just a suggestion. Your borrowed N idea doesn't hold up as the OP said the RCD can be reset once the motor is running. Its just the start up that trips it.

westward10 · Oct 5, 2017

Do you know the exact type of split phase motor it is.

marconi · Oct 5, 2017

My early thinking is along the following lines:

Worth checking the motor IR and its supply IR to confirm all is well even though it is a new motor and new installation.

1. DOL motor start - large transient current through L and N.
2. TT system so N not hard wired to E in installation so N can change in potential with respect to E/cpcs.
3. 1 above means all L conductors throughout installation will experience a transient drop in potential with respect to E/cpc. This will cause a current to flow through distributed capacitance between L conductors and E/cpcs and earth mass.
4. 1 above also means all N conductors will experience a transient rise in potential with respect to E/cpcs and body or earth. Again this will cause a current to flow through distributed capacitance between all N conductors and E/cpcs and earth mass.
5. The net result of 3 and 4 is an unbalanced current through the RCDs, and in the case of one of them sufficient for it to trip. That RCD may also be 'biased' already towards tripping because of earth leakages in the final-circuits it protects.

What to do?

a. Can you discover insulation resistance faults on the tripping RCD's final circuits? Or damp, dirt, dust, detritus near live conductors which can be cleaned away.
b. Check all L and N connections are well made and low resistance. Poor connections can create transients only noticeable when high currents flow through them.
b. Can the installation be converted to TNC-S to tie N and E/cpcs?

SparkyChick · Oct 5, 2017

Just out of interest... is the MCB for the motor that causes the tripping next to the RCD that trips?

Marvo · Oct 5, 2017

Some of the cheaper RCD's can be twitchy with phase angle change from non-linear loads like motors.

Another possibility is the motor is causing a voltage spike that's being sunk to earth by surge arrestors on a completely different circuit which can get RCD's all emotional especially if they're earth referenced.

Before you get complicated, the first thing is to to do the usual barrage of tests on the RCD, IR all the circuits fed from the RCD, check for borrowed /crossed neutrals etc. Also IR test the motor @ 500v and spin the motor by hand whilst your IR testing it.