Discuss Very strange RCD tripping problem on TT system. Need advice in the Electrical Forum area at ElectriciansForums.net

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I'm in France and this is my first post, so apologies if this is in the wrong section. Here is the problem.

I have a client, with a swimming pool and heat pump for the pool, but their main supply keeps tripping. The connection is:

* Supply companies incoming feed, three-phase, limited to 30A per phase, with 500mA RDC (supply companies)
* 60 mtrs underground (all three-phase) feed to second 500mA RCD (the customers, installed before my time)
* 40 mtrs to the first Consumer unit and DP. At this DP, there is the TT earth spike and a 20A, 4 pole MCB, which is the supply to the swimming pool area
* 80 mtrs to the pool house which has its own Consumer unit. Here there is a three-phase 30mA RCD, and MCB's for the pool pump (mono phase) which is on a simple timer, heat pump (mono phase), a salt to chlorine system, and a few sockets and lights

Here is the problem. The system keeps tripping out, but it trips out:-

a) As the timer switches the pool pump off
b) It trips all the way back to the 500mA RCD. The MCB's at the pool house, or DP do not trip, nor does the RCD at the pool house

I have disconnected the earth supplied from the DP and installed a local earth at the pool house, with no noticeable effect.

Any suggestions will be gratefully received.
 
1. Can you say more about the 'simple timer'? Is it single or double pole? What make and type? Does it switch the pool pump current or does it control a contactor?

2. Has the problem always been present? Is it a new installation?

3. Is it 3 phase 4 wire to the pool?

4. What kind of ground is the earth electrode driven in to? Dry or moist?

5. What equipotential bonding is installed in the pool house and house?

6. What kind of main supply? TNC-S, TN-S or TT?

7. Is it metal water and gas pipes between house and pool house?
 
Maybe I didn't make this clear. The earth (remember this is a TT system), is at the DP and was linked on to the pool area.

Rather than use this, a new earth was installed at the pool area.
 
Maybe I didn't make this clear. The earth (remember this is a TT system), is at the DP and was linked on to the pool area.

Rather than use this, a new earth was installed at the pool area.

Installing a separate earth will NOT stop a RCD tripping.

Give me another dislike if you want but this is fact.
 
I would be inclined to start with IR tests.
Have you carried out any testing of the 30mA RCD to ensure it is working correctly?
When you say the 500mA RCD is tripping do you mean the customer's or the supplier's.
 
It trips all the way back to the 500mA RCD. The MCB's at the pool house, or DP do not trip, nor does the RCD at the pool house
Bernie, do not get confused about RCD tripping characteristics.

You have a 500mA RCD (supply), with a 30mA RCD (consumer).
There's a misconception that the 30mA will trip before the 500mA. This is not the case. If the earthy fault is more than 500mA then the RCD that will trip will be the one with the faster trip time. RCDs can only be discriminated on the basis of time, and not current.

What you are describing is an earth fault of more than 500mA. That makes it a big earth fault btw. I agree with the above, you need someone with the appropriate test gear. unfortunately, he may only speak French.
 
Thanks for all the replies. I'll try and deal with them all one by one.

First, IR testing has not been done. My Megger is away at Fluke for recalibration and this being France, I don't expect it back for at least another 10 days. I can perhaps borrow one. Thanks for the thought, but if it is an insulation issue, why does is the tripping not intermittent when the pool pump and heat pump are off?

Second, the 500mA RCD that goes can be either. Their are two types in use here. One "instantane" and one "selectif". Where one follows another, they should be (as the names imply), the selective one as the supply companies, and the rapid one as the customers so it should normally be the customers one that goes. However (again, this being France), to get the supply company to install a selectif, is impossible, so they are both instantane and it is random as to which one goes.

Third, RCD testing. I have done 5X tests on the pool house RCD and its fine.

Fourth, the pool pump is single phase, as is the heat pump. Not my preference but it is what it is.

Fifth, back EMF from the motor. I could well be back EMF from either the pool pump or the heat pump. If it is, what are the options to deal with it?
 
The switch that turns off the motor - is it 2 pole? At switching I'm thinking the N may be already floating and creating an issue. To test this you could temp. cobble a SP switch in the L at the motor and see if this behaves the same.
 
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