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.

 

[jackhammerJIM]

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

and your point is ??????????

the rcd is working correctly .

what qualifications do you have frenchie ?

 

[frenchie6745]

To be honest if it was stated somewhere that you could or shouldnt then i would or wouldn't!
Didnt want RCD in house to trip and cause nuissance to customer not that it should due yo the quality of the installation yoy understand?
I have been told over the years that outbuilidings cannot use PME?
More than happy if they can, saves me a lot of work.

 

[frenchie6745]

Qualified and working. just never seen this happen before.
NOT a Diyer if that helps.

 

[GLENNSPARK]

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

so...whats the problem?

rite:

for a start get back round there and get that armoured off the RCD side of that board (top end).

get rid of that bloody rod in the shed.

use one of the cores (black) as the earth...take it straight into db2s MET.

use the grey as the neutral...get all cores identified top & bottom end (colours)

you have a 10mm CSA core in the SWA...so you dont need to be pulling in a seperate G/Y..do you (just before you bothered asking)...

30mA RCD in db2 for finals in there...

 

[GLENNSPARK]

come on...this isn`t hard...

 

[telectrix]

as glenn says. use 3rd core and armour ,earthed both ends, for cpc ( and bonding if required for extraneous). extend PME and forget the rod.

 

[GLENNSPARK]

To be honest if it was stated somewhere that you could or shouldnt then i would or wouldn't!
Didnt want RCD in house to trip and cause nuissance to customer not that it should due yo the quality of the installation yoy understand?
I have been told over the years that outbuilidings cannot use PME?
More than happy if they can, saves me a lot of work.

well then you got told a load of old chud there didn`t you...

 

[BES]

As glenn says earth swa of shed cable at met but not at db2

 

[Richard Burns]

As glenn says earth swa of shed cable at met but not at db2

Pretty sure he did not say that!

 

[7029 dave]

Glen, steady you will be going round there, and doing it for him lol!!.