DB2 (TT) trips DB1 TN-C-S

[frenchie6745]

Evening all

Been on here for some time now but never posted as always found the answers to my questions in older posts.

Just completing a domestic rewire with shed/office on DB2.

Installed new DB1 (split load 2 x 30mA RCDs) on new DNO TN-C-S supply. Great readings, no probs.

Run 10mm SWA to shed and installed new DB2 on a TT systm, SWA protected from DB1.

Earth spike readings are 153.3ohms. High but acceptable, if customers wallett deep enough i will install another ground spike.

Db2 supplies 2 x 20A radial ccts and 2 x 6A lighting circuits, nothing onerous there and well over the requirements needed.

When i carry out an RCD test, x1/2=good, x1 & x2 trip Db1 RCD???

Any suggestions please. What have i missed?

 

[Octopus]

Yup. Sounds like you are supplying DB2 from a RCD protected circuit on DB1. That will always happen.

 

[BES]

As murdoch says have you fed supply for db2 from rcd protected mcb at db1

 

[telectrix]

as above. also, why have you TT'd the shed/office?

 

[Octopus]

as above. also, why have you TT'd the shed/office?

Because he thinks the RCD in the shed won't trip the first one!

 

[Deleted member 9648]

Remember an RCD works by seeing an imbalance in the line and neutral current.All RCD's through which the supply passes will see an imbalance and trip.
Does beg the question as to why you have ditched a perfectly good TNCS for a rubbish TT though....although I've a feeling I'm going to deeply regret asking.

 

[BES]

Does beg the question as to why you have ditched a perfectly good TNCS for a rubbish TT though....although I've a feeling I'm going to deeply regret asking.

I totally agree ( take cover incoming)

 

[GLENNSPARK]

Evening all

Been on here for some time now but never posted as always found the answers to my questions in older posts.

Just completing a domestic rewire with shed/office on DB2.

Installed new DB1 (split load 2 x 30mA RCDs) on new DNO TN-C-S supply. Great readings, no probs.

Run 10mm SWA to shed and installed new DB2 on a TT systm, SWA protected from DB1.

Earth spike readings are 153.3ohms. High but acceptable, if customers wallett deep enough i will install another ground spike.

Db2 supplies 2 x 20A radial ccts and 2 x 6A lighting circuits, nothing onerous there and well over the requirements needed.

When i carry out an RCD test, x1/2=good, x1 & x2 trip Db1 RCD???

Any suggestions please. What have i missed?

why?...

you have stated 10mm SWA...is it a 3 core?

is there extranious in the shed?

or are you another of them that say you cant extend a PME?

 

[GLENNSPARK]

Yup. Sounds like you are supplying DB2 from a RCD protected circuit on DB1. That will always happen.

it will happen if the top end RCD...(call it RCD 1) is being subjected to leakage from other circuits its offering additional protection to....also the individual device`s characteristics being taken into account.

smells like another poor design thread to me does this...

 

[frenchie6745]

DB1 RCD protecting 3 core SWA.
3rd core CPC not connected at DB2.
DB2 25/30m away.