Strange/Unusual Fault

[mattg4321]

So there I was the other day posting on here about me going to solve faults others couldn't find and I've got one myself.

It's been a while since I've been stumped but I am now.

Symptoms - gate supply keeps tripping (when it's raining according to customer). There's a 6mm 3 core SWA running about 3-400m (yes you read that right), to a cabinet by the gates where there is a double socket inside feeding a right mess for the gates. Protected by Hager reduced height 20A RCBO.

Unplugged both sockets, IR tested cable from consumer unit to double socket.

L-N = 140Mohm
L-E = 71Mohm
N-E = 70Mohm

IR tested the plugs for the gate controls. L&N together to earth. One was 12Mohm, the other was 299Mohm. Throughout all the below I never plugged the gates back into the double socket.

With the plugs still unplugged and nothing in the double socked I went to turn on the RCBO. Took 4 goes to get it to stay in. No bang, just as if there was N/E fault there. 1/2 trip and ramp tested RCBO and both returned no result as it tripped so early/quickly. Removed outgoing cables from RCBO and all tested well - 23mA ramp test and turned on without any trouble.

Changed RCBO to rule that out and same symptoms.

Tested IR a few more times to make sure and same results. Tested outgoing cables to socket to all other cables in consumer unit to see if anything was mixed up. All good. Physically traced cables in consumer unit to check same and again all good.

I'm guessing it's something to do with the length of the cable? I can only guess that somehow I'm losing current from the line to the earth as they are run alongside eachother for such a distance? This doesn't really make sense to me though. Someone put me out of my misery!

 

[DPG]

So there I was the other day posting on here about me going to solve faults others couldn't find and I've got one myself.

It's been a while since I've been stumped but I am now.

Symptoms - gate supply keeps tripping (when it's raining according to customer). There's a 6mm 3 core SWA running about 3-400m (yes you read that right), to a cabinet by the gates where there is a double socket inside feeding a right mess for the gates. Protected by Hager reduced height 20A RCBO.

Unplugged both sockets, IR tested cable from consumer unit to double socket.

L-N = 140Mohm
L-E = 71Mohm
N-E = 70Mohm

IR tested the plugs for the gate controls. L&N together to earth. One was 12Mohm, the other was 299Mohm. Throughout all the below I never plugged the gates back into the double socket.

With the plugs still unplugged and nothing in the double socked I went to turn on the RCBO. Took 4 goes to get it to stay in. No bang, just as if there was N/E fault there. 1/2 trip and ramp tested RCBO and both returned no result as it tripped so early/quickly. Removed outgoing cables from RCBO and all tested well - 23mA ramp test and turned on without any trouble.

Changed RCBO to rule that out and same symptoms.

Tested IR a few more times to make sure and same results. Tested outgoing cables to socket to all other cables in consumer unit to see if anything was mixed up. All good. Physically traced cables in consumer unit to check same and again all good.

I'm guessing it's something to do with the length of the cable? I can only guess that somehow I'm losing current from the line to the earth as they are run alongside eachother for such a distance? This doesn't really make sense to me though. Someone put me out of my misery!

I'd be surprised if that could cause enough current to flow to trip an RCBO. 15mA+ just by the coupling of adjacent cables?

 

[Jim_e_Jib]

Did you include the armour in the IR tests?

 

[mattg4321]

Armour was connected to cpc and earth at all times

 

[timhoward]

Is it a metal double socket?
What I don't think you've mentioned is taking front off double socket, but see exactly what you mean about the mix of IR tests and RCBO tripping.
Is it an RCBO with a functional earth connection?

 

[snowhead]

Change the socket, or put the cable ends in a block and power up.

 

[stevethesparks]

I wonder if you may have some capacitance issues over such a run? Just a thought.

 

[Lucien Nunes]

Yes, capacitive leakage is a thing and on long cables it's an important thing. This case looks borderline.

Manufacturers don't usually publish capacitance for small LV power cables in their data, but at some point I clearly managed to get data for 2.5mm SWA as I wrote about it in this thread (although did not include the link mentioned there):

Capacitive Inductance | on Free Electrical Advice - Electricians Forums

Find Capacitive Inductance Advice and Help. How-to Capacitive Inductance in the UK Electrical Forum advice boards on ElectriciansForums.net | Free Electrical Advice - Electricians Forums: Electricians' Talk Forums. Capacitive Inductance on ElectriciansForums.net Electrical Advice Forum.

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My figure of 0.25μF/km was for one core to all the others, whereas with a 3-core on single phase, leakage to the neutral core does not create residual current, so the effective L-E capacitance for our purposes is somewhat lower. However, the cable here at 6mm² has conductors of larger surface area but probably not spaced proportionately further apart, so the capacitance per metre will be higher.

If we took a wild guess at 0.3μF/km L-E effective, the cable will contribute
230 * 2 * pi * 50 * 0.3^e-6 = 22mA of leakge per km

Hence 22 * 350 /1000 = 8mA for a 350m cable (very roughly, since we don't have the actual capacitance for that cable size). That is the leakage at 50Hz, however, when closing the RCBO on the peak of the mains waveform, the fast risetime voltage step might pass a big enough slug of charge to earth to trip the RCBO straight back off. The leakage might be somewhat higher due to harmonics and noise on the supply, too, as the capacitance to earth tends to filter them off. I could imagine a situation where switching large loads on the same supply caused spikes of many mA through the cable capacitance.

If the customer is correct that it trips more when raining, there might be moisture ingress into the cable which, if it surrounds the line core insulation, further increases the capacitance. Alternatively, the tripping when already energised might be a combination of capacitive and resistive leakage (from other causes), while the difficulty resetting is due to the transient voltage across the capacitance.

100mA RCBO at the source for fault protection and 30mA at the destination for additional?

 

[timhoward]

L-N = 140Mohm
L-E = 71Mohm
N-E = 70Mohm

I'm thinking out loud here...
Assuming armour is earthed both ends.
With E and armour earthed you have same effect as a shorter cable and reduced (normal) resistance.
IR is inversely proportional to the cables length.

All that to say shouldn't the IR measurements involving E be higher than the L to N reading?
EDIT - ^^complete rubbish^^ penny has dropped now....

 

[Lucien Nunes]

I don't follow you, but the individual resistances core-core and to SWA are governed by the bulk resistivity of the insulation and filler materials, and the area and thickness of each. With the large area of SWA in contact with the filler, and this not necessarily having high insulation properties, one might expect the resistance from each core to be lower to the SWA than to another isolated core, since only one layer of insulation is involved instead of two. If the CPC core is strapped to the SWA, I would expect results much as the OP has measured.

OTOH, these resistances are all up in the 'ignore' category. They would need to be a thousand times lower to be playing any significant role in the RCBO tripping.

 

[mattg4321]

Great post as usual @Lucien Nunes

It seems I was on the right track then. I had been thinking of 100ma at source and 30ma at gates on the way home as a possible solution.

 

[timhoward]

With the large area of SWA in contact with the filler, and this not necessarily having high insulation properties, one might expect the resistance from each core to be lower to the SWA than to another isolated core

Thanks. That was the main detail I didn't take into account at all. It was also completely illogical to compare normal resistance changes due to csa with the general rule of IR getting shorter as a cable gets shorter. Total brain fail really!

 

[littlespark]

Monday night brain click here, between moving furniture and packing a bag….
(We are going for a few days holiday, she hasn’t chucked me out just yet!)

does the circuit need any rcd at source end at all? Armoured cable

 

[Pete999]

I'd be surprised if that could cause enough current to flow to trip an RCBO. 15mA+ just by the coupling of adjacent cables?

Call someone with the correct test gear to test your installation

 

[DPG]

Call someone with the correct test gear to test your installation

Lol. Not my installation Pete. I was just thinking aloud!