Discuss Strange RCD tripping scenario in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Rkidcd

-
Reaction score
2
Any suggestions as to possible causes of RCD tripping in the below domestic scenario would be helpful:

Origin:
single phase TT supply into an external cabinet housing cutout and our installed 100A 100mA Type S 2Pole RCD. Earth Rod has also been sunk here.

House:
3c SWA supply to house from above to a TP&N board with single phasing kit. All circuits from this board are on 30mA RCBO’s.

We have been turning circuits on once tested over the course of last 2 weeks and the board has sat with all but 2 circuits on for nearly a week without issue but all of a sudden today we have had nuisance tripping at the RCD in the cabinet.

We have insulation tested the sub-mains again as a result of this and everything seems fine. So at the minute I’m unable to find the cause, none of the RCBO’s have tripped at any point.

Any suggestions?
 
I reckon it'll be a neutral to earth fault on one of the final circuits, I've know this phenomenon to be undetected by RCBOs, tricky to explain exactly why though.
 
Sometimes in particular TT systems a neutral to earth fault may not cause sufficient imbalance due to the neutral path having lower resistance than the path to earth. So you could have a dead short neutral/earth fault but the majority of current flow stays with the neutral path which may not create sufficient imbalance to trip the device.
 
Thanks Westward, saved me the effort. Also why if you snip a 'dead' cable on a TNCS or TNS system the relevant RCD will generally 'trip' often resulting in explaining to a customer that you're not an idiot and you haven't cut a live cable. But 9 times out of 10 you'll get away with it on a TT.
 
Sometimes in particular TT systems a neutral to earth fault may not cause sufficient imbalance due to the neutral path having lower resistance than the path to earth. So you could have a dead short neutral/earth fault but the majority of current flow stays with the neutral path which may not create sufficient imbalance to trip the device.

an S type?

Not totally convinced BUT it could be accumlated leakage ....... especially with RCBO's ........

Wonder if the OP has an earth clamp leakage meter?
 
Thanks, most of the above makes sense to be honest - you do in fact get away with it on TT majority of time in like you say.

However why doesn’t the RCBO trip ahead of the time delayed?
 
an S type?

Not totally convinced BUT it could be accumlated leakage ....... especially with RCBO's ........

Wonder if the OP has an earth clamp leakage meter?

I haven’t but sounds like it could be a good purchase
 
Thanks Westward, saved me the effort. Also why if you snip a 'dead' cable on a TNCS or TNS system the relevant RCD will generally 'trip' often resulting in explaining to a customer that you're not an idiot and you haven't cut a live cable. But 9 times out of 10 you'll get away with it on a TT.
You need to ditch the neutral or you provide an alternative path for the neutrals of all the circuits still in use.
 
Most obvious to me would be leakage current not enough on each circuit to trip 30mA RCBO but accumulatively unless the 100mA is tripping out much lower than it should.
 
Most obvious to me would be leakage current not enough on each circuit to trip 30mA RCBO but accumulatively unless the 100mA is tripping out much lower than it should.

I’ve thought about trying a 300mA - there’s over 20 Rcbo’s

I haven’t ramp tested but I have had 2 different manufactured RCD’s in today, both tripping
 
You'd have to see if the disconnection times could be met based on Ze, have you I.R tested every circuit L-E and N-E ?
 
You say it's been ok for 2 weeks, then the nuisance tripping has started. You also mention the cabinet with the 100mA RCD is external. Is this tripping happening at the same time of day/night? Have the atmospheric conditions changed? I'm wondering if the cause may be condensation related.
 
You say it's been ok for 2 weeks, then the nuisance tripping has started. You also mention the cabinet with the 100mA RCD is external. Is this tripping happening at the same time of day/night? Have the atmospheric conditions changed? I'm wondering if the cause may be condensation related.

It could be something as simple as this Tony
 

Reply to Strange RCD tripping scenario in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock