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!!.

 

[GLENNSPARK]

what i wouldn`t mind knowing at this point is just who the hell told him you cant extend PME?

really iresponsible...

 

[GLENNSPARK]

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

he can have guidance...but its up to him to apply it...

applying a bit of common sense wouldn`t go amis either...such as putting 2 30mA RCDs in series...

see it a lot in here dont we boys...

 

[jackhammerJIM]

lots of hear say GLENN . people are too lazy to look the information up themselves .

do you own a current regs book frenchie ?

have a look through the special locations for where PME is prohibited .

 

[cotty]

iresponsible... ?? missing something??

 

[GLENNSPARK]

iresponsible... ?? missing something??

it seems to me that he has been told that you cant extend PME...full stop.

not true...is it..

 

[Deleted member 9648]

The myth persists.......thought we'd nailed the coffin lid on that one many moons ago.

 

[GLENNSPARK]

The myth persists.......thought we'd nailed the coffin lid on that one many moons ago.

old yorkshire proverb:

you cant educate pork

 

[cotty]

old yorkshire proverb:

you cant educate pork

You can spit roast it though! And yes, you are right..

 

[GLENNSPARK]

You can spit roast it though! And yes, you are right..

here`s another:

you cant plant big trees in small gardens

 

[telectrix]

here's another :always pi$$ over the leeward side.

 

[BES]

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

Pretty sure he did not say that!

I know he didnt. I did
Meant to say no need to earth swa of cable at db2

 

[GLENNSPARK]

here's another :always pi$$ over the leeward side.

works for high TT as well...

as in:...come down you bitch..

 

[cotty]

I know he didnt. I did
Meant to say no need to earth swa of cable at db2

No! Must agree there! I'd run a separate 10mm bonding with surface trunking and a spot of 'conduit' maybe!!

 

[telectrix]

i'd use 3 core SWA. armour as cpc and 3rd core for bonding if required.

 

[Engineer54]

I'm just wondering why he ran a 3 core 10mm cable and didn't use the the third core?? So why bother running a 3 core cable in the first place??

As far as the ''You can't extend a PME earthing system'', it's mostly down to the P**s Poor college lecturers you have in these colleges and training centers these days, that continue to perpetuate this MYTH!!

 

[not clever]

Nothing wrong with what he's done (except the rcd's in series) if that's how he has designed it and is happy to waste money on 3 core but not use the 3rd core to extend the pme.
Me I'd have saved some bunce and gone 2 core swa, split the tails at the house into switch fuse out in 10mm on to outbuilding small db with rcbo's

Frenchie I'm in Andover tomorrow if you want me to sort it for you.

 

[cotty]

I'm just wondering why he ran a 3 core 10mm cable and didn't use the the third core?? So why bother running a 3 core cable in the first place??

As far as the ''You can't extend a PME earthing system'', it's mostly down to the P**s Poor college lecturers you have in these colleges and training centers these days, that continue to perpetuate this MYTH!!

or old farts not willing to evolve! Whoops! ... Has he been provoked?...

 

[Silly Sausage]

here's another :always pi$$ over the leeward side.

You can get grief from Eddy currents though!

 

[Engineer54]

or old farts not willing to evolve! Whoops! ... Has he been provoked?...

Nope is just P**s Poor college lecturers that instead of teaching if you like, just the facts, they incorporate their own ill found misconceptions!! The same goes it seems, for RFC's now being touted as dangerous!! lol!!

 

[GLENNSPARK]

Nope is just P**s Poor college lecturers that instead of teaching if you like, just the facts, they incorporate their own ill found misconceptions!! The same goes it seems, for RFC's now being touted as dangerous!! lol!!

well eng:

i never listened to any of the old drivvel about RFCs then....and i wont now either...
and if young robert who comes out with us starts being indoctrinated by any college slavver about the `dangers` of a properly designed RFC...or how you cant extend a PME then i will be on hand to lead him back into the light...

 

[ipf]

well eng:

i never listened to any of the old drivvel about RFCs then....and i wont now either...
and if young robert who comes out with us starts being indoctrinated by any college slavver about the `dangers` of a properly designed RFC...or how you cant extend a PME then i will be on hand to lead him back into the light...

And stomp down to the college to re educate the firkin board slapper.

 

[GLENNSPARK]

And stomp down to the college to re educate the firkin board slapper.

yep...i`ll grab em by`t scuf ov`t neck...

 

[Broken handle]

Nothing wrong with what he's done (except the rcd's in series) if that's how he has designed it and is happy to waste money on 3 core but not use the 3rd core to extend the pme.
Me I'd have saved some bunce and gone 2 core swa, split the tails at the house into switch fuse out in 10mm on to outbuilding small db with rcbo's

Frenchie I'm in Andover tomorrow if you want me to sort it for you.

Exactly how I would do it (and have on many occasions). At a distance of 30m from main building I would not be exporting PME but relying on TT. For your information, Frenchie, the reasons you do not want two 30mA RCDs in series are something you should be aware of when doing work of this nature.

 

[Deleted member 9648]

Exactly how I would do it (and have on many occasions). At a distance of 30m from main building I would not be exporting PME but relying on TT. For your information, Frenchie, the reasons you do not want two 30mA RCDs in series are something you should be aware of when doing work of this nature.

Why?,,

 

[Octopus]

Why?,,

My thoughts exactly. My shed is about 30m away and I've exported the earth. Why ditch a really god earth for a unreliable one!!

 

[Broken handle]

Why? Because although at 30m it is far enough away from the house and (probably) any extraneous metal, with a TT I have a separate earthing system which is, if you like, under known control conditions and not subject to problems at the suppliers end. TT'ing I agree is not really an issue where step-voltages are highly unlikely but I generally will always TT an outbuilding combined with 30mA RCD.

Sorry, above should say "Exporting PME....."

 

[Engineer54]

Why? Because although at 30m it is far enough away from the house and (probably) any extraneous metal, with a TT I have a separate earthing system which is, if you like, under known control conditions and not subject to problems at the suppliers end. TT'ing I agree is not really an issue where step-voltages are highly unlikely but I generally will always TT an outbuilding combined with 30mA RCD.

Then you're a fool!! So what happens when that 30mA RCD fails then?? You've been listening to far too many scaremongers!!

 

[Broken handle]

So what happens when that 30mA RCD fails then??

All things are liable to fail. But I do not get in my van and think, better not drive today because the brakes might fail. If everything is designed correctly and works as it should then I should not be having sleepless nights about possibilities of failing components. Once my test & inspection cert. is done (and BTW, I always take photographic evidence of my installs :smiley2 then the installation is as it is and so long as i can stand up in court and prove everything was done within regs. then I am happy - even if I might agree with the view "in hindsight.....":wink5:

 

[Engineer54]

In other words Sod the customer you've taken money from!! Magic!!
Any electrician that's fool enough to ditch a perfectly good TN Earth in favour of anything like the totally unreliable recommendations of BS 7671 regarding TT systems, needs Certifying!!

 

[Octopus]

In other words Sod the customer you've taken money from!! Magic!!
Any electrician that's fool enough to ditch a perfectly good TN Earth in favour of anything like the totally unreliable recommendations of BS 7671 regarding TT systems, needs Certifying!!

Along with the install!