Max Ze for TT system

[MBEINST]

Hi Gents and ladies,

I have been asked to quote on installing a couple of extra socket on a TT system.

When checking the existing installation my Zs readings on the ring are coming out at 39.8 Ohms (most remote). This is unacceptable (table 2d OSG). I then checked Ze readings which are coming out at 37 Ohms.

I'm relatively new to the game and never dealt with a TT system before, so went back to my books and have an hand written note in my 2391 notes stating 21 Ohms for TT system? Can't remember why I wrote this but assume it referred to the max Ze allowable for TT system. However OSG under Earth Electrode Resistance states 'if being used with an RCD should not exceed 200 Ohms'. I'm dealing with an old installation and no sign of RCD protection. Can't convince the customer to upgrade Consumer Unit.

It looks to me like this is going to turn into a major job and obviously I don't want to quote for a couple of extra sockets to find out that I need to put in another rod.

Any help to nurse this TT virgin through would be greatly appreciated.





'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

 

[Octopus]

If you can't convince the client to change the CU, how are you going to provide RCD protection? Is the bonding in place?

Back to the TT Ze, 200 ohms is good enough - but the lower the better so 37 ohms is OK IMHO.

 

[Engineer54]

If you haven't any existing RCD protection in place, then any new socket outlets will either need to be RCD outlets or a separate circuit supplied at origin via a supplementary RCD 1 way CU or the like!!

200 ohms or anything like it, is NOT good enough, it never has and never will, no matter what it say's in BS7671. 21 ohms is the maximum for a DNO substation, but you'll be hard pressed to see/find any substation with an Ra approaching anywhere near that high a value, it'll be more like 1 ohm or less!!

If you can't convince your customer to upgrade the existing unprotected CU to a modern protected CU, then i fear you'll not convince them on providing RCD protection on these additional sockets either, in which case you should simply walk away....

 

[slocm3105]

If you haven't any existing RCD protection in place, then any new socket outlets will either need to be RCD outlets or a separate circuit supplied at origin via a supplementary RCD 1 way CU or the like!!

200 ohms or anything like it, is NOT good enough, it never has and never will, no matter what it say's in BS7671. 21 ohms is the maximum for a DNO substation, but you'll be hard pressed to see/find any substation with an Ra approaching anywhere near that high a value, it'll be more like 1 ohm or less!!

If you can't convince your customer to upgrade the existing unprotected CU to a modern protected CU, then i fear you'll not convince them on providing RCD protection on these additional sockets either, in which case you should simply walk away....

Well said that man!!!

 

[Deleted member 9648]

Hi Gents and ladies,

I have been asked to quote on installing a couple of extra socket on a TT system.

When checking the existing installation my Zs readings on the ring are coming out at 39.8 Ohms (most remote). This is unacceptable (table 2d OSG). I then checked Ze readings which are coming out at 37 Ohms.

I'm relatively new to the game and never dealt with a TT system before, so went back to my books and have an hand written note in my 2391 notes stating 21 Ohms for TT system? Can't remember why I wrote this but assume it referred to the max Ze allowable for TT system. However OSG under Earth Electrode Resistance states 'if being used with an RCD should not exceed 200 Ohms'. I'm dealing with an old installation and no sign of RCD protection. Can't convince the customer to upgrade Consumer Unit.

It looks to me like this is going to turn into a major job and obviously I don't want to quote for a couple of extra sockets to find out that I need to put in another rod.

Any help to nurse this TT virgin through would be greatly appreciated.





'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

With the Zs values you state and no RCD as it stands the existing install has no earth fault protection whatsoever, and you should advise the client that a dangerous situation exists.Do not carry out any work without upgrading.
If the install has to remain TT then the ideal solution is a DB with a 100ma S type main switch and RCBO's....less ideal is a standard dual RCD board,but it would still comply. The maximum Zs where final circuits are 30ma RCD protected is 1667 ohms...a maximum Ra of 200 ohms is advised.
Even better convert it to TNCS if available

 

[Engineer54]

The maximum Zs where final circuits are 30ma RCD protected is 1667 ohms...a maximum Ra of 200 ohms is advised. Only by plonkers!!
Even better convert it to TNCS if available

Now that's GOOD if not the best advice, where a PME supply is available....

 

[D Skelton]

200 ohms or anything like it, is NOT good enough, it never has and never will, no matter what it say's in BS7671. 21 ohms is the maximum for a DNO substation, but you'll be hard pressed to see/find any substation with an Ra approaching anywhere near that high a value, it'll be more like 1 ohm or less!

Yes, as this guy said, 200 ohms is rubbish and 20 ohms is better, although as professional electricians we have no idea why as a fault occuring with both these values of Ra would trip both a 30mA and a 100mA RCD???

Dogma at best

 

[i=p/u]

No sense in wasting earth spikes if you not going to get what you want out of it .

 

[LeeH]

This is what I don't understand ^^^^^

Logic dictates that 20 is better then 200 but the RCD will still operate within 300ms?

 

[i=p/u]

Weather you get it to 2 ohms you still relying on rcd for circuits within board

 

[MBEINST]

Thanks for the feedback fellas.

Although work is tight and I need to keep the wolf from the door like everyone else. I was going to walk away unless I can convince the customer to upgrade his consumer unit.

I have been fitting the Denmans fully loaded RCBO units of late and think that this would be the solution. Obviously it would be great to change the earthing arrangement to TN-C-S but surely this would be done by the supply company and would cost an arm and a leg.

:uhoh2:

'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

 

[telectrix]

This is what I don't understand ^^^^^

Logic dictates that 20 is better then 200 but the RCD will still operate within 300ms?

RCD on a TT must operate within 200mSec. this is achieved by a Zs value of <1667 (30mA RCD).

 

[Engineer54]

Yes, as this guy said, 200 ohms is rubbish and 20 ohms is better, although as professional electricians we have no idea why as a fault occuring with both these values of Ra would trip both a 30mA and a 100mA RCD???

Dogma at best

When i first came into this industry the recognised max for a Roded earth electrode was 10 ohms, FACT!! Never saw one anything like at that value mind, all were generally in the lower single figures.

So now were being sold a complete crock crap that 200 ohms is the max figure to aim for!! If for no other reason, a 200 ohm earth electrode, or anything remotely like it will ''NOT'' provide a stable TT system.

 

[Engineer54]

Thanks for the feedback fellas.

Although work is tight and I need to keep the wolf from the door like everyone else. I was going to walk away unless I can convince the customer to upgrade his consumer unit.

I have been fitting the Denmans fully loaded RCBO units of late and think that this would be the solution. Obviously it would be great to change the earthing arrangement to TN-C-S but surely this would be done by the supply company and would cost an arm and a leg.

:uhoh2:

'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

It shouldn't cost a penny, the old regional boards used to convert TT systems for nothing, in fact that was one of the main reasons the electricity boards invested so much cash into PME'ing overhead and later failing underground cables, to provide homes and businesses with a stable means of earthing!!

Seems now the private DNO's think it's fine to charge for this once totally free service. But seeing as it takes about 2 to 3 min's to convert (depending on the service head) it shouldn't cost an arm and a leg!!

 

[Engineer54]

Weather you get it to 2 ohms you still relying on rcd for circuits within board

It may not trip a protective device out in the required time but it WILL take out protective devices!! On the other hand the 200 ohm crap you think is fine will be whistlin dixie for ever and a day....

 

[LeeH]

RCD on a TT must operate within 200mSec. this is achieved by a Zs value of <1667 (30mA RCD).

Of course, my bad.

 

[MBEINST]

Hope this isn't a stupid question,

Given what has already been discussed regarding TT systems, how is it possible to get acceptable Zs readings which will comply with table 2D OSG.

 

[Guest 004]

Hope this isn't a stupid question,

Given what has already been discussed regarding TT systems, how is it possible to get acceptable Zs readings which will comply with table 2D OSG.

Simple you can't

 

[D Skelton]

When i first came into this industry the recognised max for a Roded earth electrode was 10 ohms, FACT!! Never saw one anything like at that value mind, all were generally in the lower single figures.

So now were being sold a complete crock crap that 200 ohms is the max figure to aim for!! If for no other reason, a 200 ohm earth electrode, or anything remotely like it will ''NOT'' provide a stable TT system.

I got a bite! A big one! C'mon lads, get the landing net out ready, this one's gonna be a wriggler!

 

[Guest 004]

Was going to reply, then realised how bigoted some posters are, so will just watch and read with delight.