RCD problems 2

[carlosmelim]

In one of our panels the main switch disconnector is a RCD.
It has been working fine for almost three months but suddenly it started to trip occasionally. I switch it on again and everything works perfectly for some hours,

The strange part is that I´ve seen it trip with every circuit of the panel disconnected ( no load ).
I even performed an isolation measurement on the busbar and it was OK.
Normally the RCD is regulated to 300mA. I have raised the regulation to 10A and it still trips with no load.
I am convinced that there is a malfunction of the RCD itself but someone told me that it could be an earthing problem.

Any suggestions?

Carlos Melim

 

[RajT]

Carlos,

It is a misconceptionthat RCDs trip without any earth faults or due to faulty mechanism.
If the unit is tripping intermittenly and assuming that that there is no earth (even intermittent)faults on any of the out going circuits then you should try the following.

Do not increase the trip rating to 10amps as it then defeats the purpose of installing the RCD in the first place.
Go back to the orig unit with a setting of 300mA but change it to a unit with a delayed tripping of say 100ms.
ie install a unit which 300mA/100ms.
The delay will take care of any high magnitude spikes which often incure when switching inductive loads.

 

[carlosmelim]

RajT, let me clarify the situation:

The RCD it´s always the same. It´s a SIEMENS 3VF equipped with a so-called diferential module. This module accepts regulations from 10mA up to 30A with up to 200ms delay. In Portugal we normally use 300mA for common purposes and 30mA to outlets.

I have lowered the regulation to 30 mA in order to detect any possible faults. Normally an intermitent defect with 300mA turns to steady defect with 30mA. In my panel with 30 mA the symptoms remain the same even with 100ms: intermitence trips.

Please remember that my RCD trips with every single circuit of the panel disconnected ( absolutely no load or spikes). And with 10A regulation it still trips.

Looking forward to hearing from you,

Carlos Melim

 

[busbar]

Intermittent problems can be agonizing. Long shot—is it possible that some wiring was reconfigured so that a local grounded-neutral conductor is being shared/used from a circuit originating from another feeder/panelboard? This manifested as intermittent, severe monitor distortion in one case, and was located with a portable 3200A current transformer {and DMM} clipped around the feeder-conductor bundles, less any associated equipment-ground cables.

 

[RajT]

Carlos,
Based on what you say, I have no answer other then perhaps changing the unit out altogether.

 

[lyledunn]

Carlos,
Is your unit 4pole? Does it have loss of neutral protection, viz; a curly wire connecting to the earth bar?
Your problem could be a myriad of things but in the first instance I am sure Siemens will provide a trouble shooting guide.
RCDs are a welcome development and a huge improvement on the old voltage operated trips. Nonetheless, as an upstream device it can be notorious. I have come across installations protected by a 300ma rcd with 100ms time delay used as the main isolator only to find that there are no downstream devices with which they are discriminating thus rendering the time delay superfulous.
I take it that this is a rcd and not an rcbo? The latter will trip on no load providing the ambient temperature is in excess of circa 40deg Cent.
I go with Busbar. My bet, apart from a faulty unit is the old neutral earth scenario. Especially sensitive are the loss of neutral guys, even with all loads disconnected neutral to earth currents can circulate and operate the tripping mechanism. Rather you than me buddy! Regards,

Lyledunn