sub main supply time delay trip 100ma 100A s keeps triping

[markbriffitt]

Hi Can anyone help ? i have a job where there is a tt supply and a swa sub main feeding a dist board with 13 final circuits , each final circuit is protected by a RCBO , all rcbos ok and not triping but the time delay trip seems very sesitive and keeps triping out , it tests ok no trip at half value , and trip at 70ma with all circuits turned off . circuits have good insulation resistance though there is a question mark over cpc continuety as some lighing cables have no cpc ,
also there is two lighting circuits which test out ok but if i try and energise both the supply trip goes out , i can power up either one and they are fine but not both together ?
any ideas would be most aprechiated .
Mark

 

[Guest77]

Sounds like there is a neutral linked to both lighting circuits somewhere in the circuitry for that issue between the 2 lighting circuits.

 

[Darkwood]

What is the measured leakage of the Submain under normal use?
Is there a lot of electronic loads with designed leakage?
The lighting maybe just tipping it over the edge when used, don't forget the 100mA is seeing the combine leakage of all the final circuits so even a small leakge of 5ma per circuit would get it near its 70mA tripping point...

Or as POST 2 suggests.

 

[Badged01]

What happens when you switch all the circuits off and just put the two lighting circuits on?

 

[Welchyboy1]

Can you install a 300mA rcd s type in its place if functional leakage is the issue? 70mA is on the sensitive side i would say

however its looks as though you do have a fault of some type present, are the unearthed circuits supplying class 1 equiptment at any point?

also are the rcbo double pole?

im surprised the individual rcbos dont simultaneously trip as well as your main rcd if you have a lighting interlink, can you IR test between your lighting circuits to check for this

 

[nickblake]

I would check the lighting circuits especially if you have external lights on them, could be water in a light thats causing it

 

[Richard]

We had a really weird fault some time ago. 100ma Time delay RCD tripping when a large water pump fed from a different phase through 2 separate RCD's (which held fine) kicked in. All wiring accessible and inspected. Explain that!

 

[Richard Burns]

We had a really weird fault some time ago. 100ma Time delay RCD tripping when a large water pump fed from a different phase through 2 separate RCD's (which held fine) kicked in. All wiring accessible and inspected. Explain that!

A neutral earth fault on the circuits protected by the time delay RCD will cause this. A very minor part of the current may flow through the NE fault but the high current from the pump means that the very minor proportion of current is above 100mA and trips the RCD.

 

[Richard]

A neutral earth fault on the circuits protected by the time delay RCD will cause this. A very minor part of the current may flow through the NE fault but the high current from the pump means that the very minor proportion of current is above 100mA and trips the RCD.
View attachment 29506

Agreed, however pump Db supplied by one 100mA type S then 30ma Rcd arrangement fed from the incomer.

When pump starts, another 100mA type S fed from a different phase supplying different equipment trips.

Maybe what you say has answered this, however i'm struggling to apply it in this instance!

Thanks