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

[DaveyD]

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

 

[RogertheBodger]

Not sure your layout is clear Davey.

Is the 100mA TD RCD up front of the whole installation? and where are the other RCD's?

 

[IzzyS]

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?

 

[DaveyD]

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.

 

[DaveyD]

... Sorry ramp test. No not done as not on the job today and it all tested ok so I was not looking for faults. Will ramp when I get back.

 

[RogertheBodger]

So the old house board only has the 100mA TD RCD in front of it?

 

[DaveyD]

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.

 

[Engineer54]

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...

 

[DaveyD]

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.

 

[Knobhead]

How is the studio fed from the S type? Is there a cable involved?

 

[DaveyD]

How is the studio fed from the S type? Is there a cable involved?

SWA to outbuilding/studio ...

Is this what you mean? It was hiding from you Tony in post #1

 

[Johnuk]

wait till you get to site, most builders tools are old and had it, so its most likely his grinder

 

[DaveyD]

wait till you get to site, most builders tools are old and had it, so its most likely his grinder

OK John, thanks for your reply.

 

[Engineer54]

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?

With just a one metre depth, Yes you could say that!!

 

[DaveyD]

With just a one metre depth, Yes you could say that!!

They are 4ft each, I stated there were two coupled together - thats more than a meter <2.5m. What are you suggesting E54? I'm not offended by the way I just don't get where you are going with this? lol