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!

 

[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!