100mA delayed RCD tripping before the 30mA at DB. TT System | on ElectriciansForums

Discuss 100mA delayed RCD tripping before the 30mA at DB. TT System in the UK Electrical Forum area at ElectriciansForums.net

DaveyD

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Hello

Just not on the job at the moment but the builder called. He plugged his small angle grinder in to a new circuit and it took out the RCD at the supplying DB not the 30mA at local CU.

Here's the set up:


TT . 100A DNO fuse

House CU old and full so split tails into small CU with 100A/100mA time delay RCD feeding a CB . SWA to outbuilding/studio and duel RCD CU.

upstream RCD 100mA Timed x1 at 310ms

Both downstream RCD's 30mA checked as perusual and passed on all seven tests.

Electrode at house and new electrode at outbuilding/studio (New at 1.87 Ohms).

My question is ... why has the time delay rcd tripped and not the 30mA? Thank you in advance for your time.

Regards, Dave
 
Have you done IR tests?
Have you tried a ramp test of the time delayed RCD? Could be that there's some leakage current, but not enough to trip it until the grinder is plugged in. For example 80mA leakage + another 10mA from the grinder causing it to trip, but not taking out the 30mA ones.

And how did you get 1.87 Ohms on TT ? Is it a rod, or something else?
 
Hi. Thanks for your replies guys. It's a upfront 100mA timed and the studio cu is a duel board 2 x 30mA split load.
All insulation resistance tests plus 500Mohms.
Electrode is 2 x 3/8s coupled.
 
The old house board has 100mA on all circuits and a further 30mA on selected. Typical old setup on tt.
This is not tripping.


I have split the tails into a new DB with 100mA delay rcd and a breaker to protect the submain to the studio.
 
Electrode is 2 x 3/8s coupled

Who put those in then?? That ground, must be more festering manure than soil!! Less than 2 ohms on a couple of short twigs, i doubt it!!

As for the S type 100mA RCD tripping before the 30mA standard RCD's, something is very wrong wouldn't you say? Time to get your test gear out, and find out why...
 
Who put those in then?? That ground, must be more festering manure than soil!! Less than 2 ohms on a couple of short twigs, i doubt it!!
Sorry my mistake 5/8 ths. I put them in. To be fair I only tested with a loop tester but ZS tests showed expected results too. What is the problem? Too LOW for you E45?

As for the S type 100mA RCD tripping before the 30mA standard RCD's, something is very wrong wouldn't you say? Time to get your test gear out, and find out why...
Thank you Sherlock :) The plumber agrees! I have tested but as explained I'm not on that job today and just wondered what others felt would make it trip upstream. Like a forum conversation. I will ramp when I get back there and replace if needed.
 
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