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!

 

[Pretty Mouth]

It’s bad practise to add RCD to comply with ADS ,this would not work on a Line-Neutral fault . This would be a departure from the regulations which does not improve safety , its better to run a bigger cable or parallel cables or even an extra R2 (cpc)

RCD for fault protection is perfectly compliant. L to N faults do not need to disconnect within stated disconnection times, only L - E. 411.3.2.1

 

[Mikegh]

When you're IR testing the connected loads at the outlet

How can you be sure you're actually getting everything

Some stuff may not be in-circuit until aftehe supply is energised


Ime SWA faults show on 500v or 1000v , could be wrong though but usually depend on the IR tester to find an issue on fixed wiring

 

[Pete999]

When you're IR testing the connected loads at the outlet

How can you be sure you're actually getting everything

Some stuff may not be in-circuit until aftehe supply is energised


Ime SWA faults show on 500v or 1000v , could be wrong though but usually depend on the IR tester to find an issue on fixed wiring

Are you testing just the installation, or the power consuming equipment? two different testing procedures ime

 

[Mikegh]

Are you testing just the installation, or the power consuming equipment? two different testing procedures ime

I was respkndiu to the first post where he said he had IR tested the SWA.eun and then IR tested.the equipment with L+N connected

I'm saying you may not be effectively checking all the wiring on the connected load as some may not be in-circuit when supply is disconnected

 

[Mikegh]

I was responding to the first post where he said he had IR tested the SWA run and then IR tested.the equipment with L+N connected

I'm saying you may not be effectively checking all the wiring on the connected load as some may not be in-circuit when supply is disconnected

Don't seem to be able to edit anything

 

[Lucien Nunes]

I agree, and it woukd be helpful to confirm the gate operator leakage current when energised. But one of the OP's key points was that it is difficult to get the RCBO to hold with just the cable connected, and the loads completely unplugged. And the cable IR tests very good. I think we're going round in circles now, there's no mystery here IMO.

 

[mattg4321]

Exactly. The rcbo was tripping at a current too low to be registered by my tester with just a cable and a socket on the end of it with nothing plugged in. Sometimes it wouldn’t even reset.

Remove cable and all ok.

 

[Pete999]

I was respkndiu to the first post where he said he had IR tested the SWA.eun and then IR tested.the equipment with L+N connected

I'm saying you may not be effectively checking all the wiring on the connected load as some may not be in-circuit when supply is disconnected

Then if it's that strange carry out two seoerate tests , the wiring and the power consuming equipment \, to try and eliminate one from the other, no good just testing it all in one procedure, any good detective will tell you to eliminate one area at a time just saying, others may call me an idiot, but been called worse. and have a thick skin.

 

[Mikegh]

I agree, and it woukd be helpful to confirm the gate operator leakage current when energised. But one of the OP's key points was that it is difficult to get the RCBO to hold with just the cable connected, and the loads completely unplugged. And the cable IR tests very good. I think we're going round in circles now, there's no mystery here IMO.

Didn't spot that , I usually shoot from the hip anyway

 

[SparkySy]

Sorry I can't remember if you said you had disconnected the socket from the swa as well, it's not moisture in that causing it? I had a gate supply that kept tripping even when it hadn't been raining. Turned out to be the socket, the enclosure it was in was ip rated but was really damp inside, it was sweating and running off the top internally down into the socket causing a N-E short occasionally.
Sy

 

[Mikegh]

Then if it's that strange carry out two seoerate tests , the wiring and the power consuming equipment \, to try and eliminate one from the other, no good just testing it all in one procedure, any good detective will tell you to eliminate one area at a time just saying, others may call me an idiot, but been called worse. and have a thick skin.

Yes obviously 2 separate procedures ?

I wouldn't combine them

Test the cable at a high voltage and equipment separately , I didn't imply differently

 

[Lucien Nunes]

He did say clearly in post #1 that the loads were tested separately and the problem was still there with them disconnected.

Presumably the socket was included in the IR tests that showed 70 / 140 megs? In which case it doesn't seem to be particularly suspect.

 

[Mikegh]

He did say clearly in post #1 that the loads were tested separately and the problem was still there with them disconnected.

Presumably the socket was included in the IR tests that showed 70 / 140 megs? In which case it doesn't seem to be particularly suspect.

He did and I may retire from posting under the influence of alcohol

 

[timbobelfast]

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!

Strange set-up, certainly no design to this circuit.

What is the design current for the gates?

You would need a B10 60898 upstream to meet disconnection times.

As it's SWA can you swap out the upstream RCBO to an MCB and fit RCD socket at the gate for the additional protection?

Certainly would not have been a cheap install for 3-400m of 3C 6 XLPE!

 

[mattg4321]

No idea re gates design current. No details present. To be honest I’m trying to stay well away from that side of things. It’s a single ram type arm that opens 1 gate.

Doubt that total load can be much. A B6 mcb would comply and might work fine? I guess the motor should only be a few hundred watts. Anyone like to make a guess?

I forgot I have a picture of the completely typical gate wiring I find whenever I look inside one of these cabinets. Utter disgrace in there.

 

[timbobelfast]

No idea re gates design current. No details present. To be honest I’m trying to stay well away from that side of things. It’s a single ram type arm that opens 1 gate.

Doubt that total load can be much. A B6 mcb would comply and might work fine? I guess the motor should only be a few hundred watts. Anyone like to make a guess?

I forgot I have a picture of the completely typical gate wiring I find whenever I look inside one of these cabinets. Utter disgrace in there.

There wouldn't be much load on the bowl of spaghetti!

You could always try squeeze in an IP rated RCD socket lol!

 

[James]

Now that is unfortunately not unusual for access control installations.

how people go home at the end of an install like that and be happy with themselves, I just don't know!!

 

[timhoward]

I think my approach would be to try and get an accurate Zs, as the length is a bit of an unknown making any attempt to introduce design into this a bit hit and miss. Current bets are it being longer than 400m based on your 4 ohm non-trip Zs.
So either dead tests, or stick any low-ish breaker in and a high current test, hence working out max breaker size up top. Fitting an RCD enclosure inside the gate box could be be the fun part.
It could always be an IP enclosure on the outside. In fact I might prefer the customer not to have to open that can of worms to reset it!