Put in a construction service drop with a Square D panel in a weather-proof box two months ago. Inside there is a 32 breaker for the cement mixer and a 16 Amp RCBO (10 mA RCD level) both made by Square D. The 16 Amp RCBO was put in to protect the construction workers.
Connected to the 16 Amp RCBO is an automatic 18 watt security light up above on the pole, and two outlets. The only connection on the outlet is a 30 meter extension cord (two conductor) to another security light and day time connection to construction tools.
Things have been fine for two months but a few days ago, the RCBO tripped off and would not reset. Trouble-shooting this I found that even with the main breaker turned off, the RCBO still will not reset. The only way to get it to reset is to disconnect BOTH the outgoing neutral and load lines on the RCBO itself. (In other words, if the main breaker is turned off, the RCBO will not reset (it immediately will trip) if the 50 meter extension cord and light are connected to the load and neutral connections of the RCBO. Disconnect them from the RCBO with main breaker still off and then the RCBO can be reset to "ready".) I measure zero volts from incoming 25mm main neutral to ground and 240 volts from incoming 25mm load to ground with main breaker on or off so no system ground faults likely.
This is quite mysterious and I had planned on using quite a few RCBOs in the house panel but I don't want to spend the money if the Square D RCBOs are not reliable. It seems as if the 30 meter extension cord is detecting radio signals and providing enough energy to trip the RCBO. I'm wondering if anyone else has experience using the Square D RCBOs. It is installed per directions and it did indeed work for two solid months with no unexpected trips. If anyone can add insight to this behavior, it would be greatly appreciated.
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Odd Rcbo Behavior
Started by T_Dog, 2012-02-06 07:01
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8 replies to this topic
#2
electau
Senior Member Posted 2012-02-06 07:41:54
First one should disconnect the conductors from the RCBO and with an insulation tester 250/500V check the insulation to earth This should be above 1 megohm. L and N .
#3
Pacificperson
Senior Member Posted 2012-02-06 07:59:46
"if the 50 meter extension cord and light are connected to the load and neutral connections of the RCBO. Disconnect them from the RCBO with main breaker still off and then the RCBO can be reset to "ready"."
It sounds like your extension cord has become waterlogged after sitting outdoors for two months. Try replacing it. Edited by Pacificperson, 2012-02-06 08:00:01. #4
T_Dog
Outside Member Posted 2012-02-06 08:11:59
First one should disconnect the conductors from the RCBO and with an insulation tester 250/500V check the insulation to earth This should be above 1 megohm. L and N . Done..... 100's of megohms, both conductors so no leakage to ground or to the other conductor. Another item that might be of interest.... The RCBO works okay until the extension cord is plugged in. But, plug the extension cord in with the main breaker on and it trips right away. Turn the main breaker off, and the RCBO will again trip off right away. We did replace the extension cord and had the same behavior. #5
Crossy
Bull Powered Member Posted 2012-02-06 08:26:10
T_dog, did you use an insulation tester or a regular multimeter to check your extension?
Does it trip with JUST the extension cord (nothing in the other end)? It really does sound like a ground fault somewhere although you could try replacing the RCBO. Is there any possibility that your workers have managed to bypass either the live or neutral of the RCBO possibly with their welder ground? 10mA on a site cable is pushing things anyway, 30mA would be more reliable (i.e. it would stay in). Can you post some photos of your box wiring, just in case you've missed something stupid 
#6
T_Dog
Outside Member Posted 2012-02-06 08:34:43
T_dog, did you use an insulation tester or a regular multimeter to check your extension? Does it trip with JUST the extension cord (nothing in the other end)? It really does sound like a ground fault somewhere although you could try replacing the RCBO. Is there any possibility that your workers have managed to bypass either the live or neutral of the RCBO possibly with their welder ground? 10mA on a site cable is pushing things anyway, 30mA would be more reliable (i.e. it would stay in). Just used a multi-meter to check the resistance, so no high voltage insulation testing done. No jury rigging done by the crew except when the welders came and connected right to the mains and disconnected after they left. No problems for two months with hundreds of tools used, welders coming and going, etc. It may be that transients from the welders damaged the RCBO. The failure happened on the day they left and disconnected two welders from the mains (before the main breaker.) Thinking through this a bit, damage from transients could be a likely cause. #7
Crossy
Bull Powered Member Posted 2012-02-06 08:39:23
Our welding friends didn't manage to swap L&N did they? Could have introduced just enough leakage to drop the RCBO.
It tripping with the power off is worrying, that incoming breaker is double pole I assume. If you can borrow or buy a cheap insulation tester (it will be handy when you do the house wiring), I would do a proper insulation test on the extension cable. Otherwise time for a replacement RCBO, get a 30mA one if you can. #8
T_Dog
Outside Member Posted 2012-02-06 08:47:12
Our welding friends didn't manage to swap L&N did they? Could have introduced just enough leakage to drop the RCBO. It tripping with the power off is worrying, that incoming breaker is double pole I assume. ......... Thanks all for the comments and ideas. Onward ever on this one.... #9
bankruatsteve
Senior Member Posted 2012-02-06 11:38:37
May not be your problem, but I had a RCBO trip until I cleaned out a hornet's nest from the ground in one of the protected receptacles and which, I assume, must have also made some kind of connection to L/N. (?)
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