TT system - Why less than 200 π ?

HappyHippyDad · May 25, 2014

I have had a read through a few threads on a TT system needing to be below 200Ω and I wondered why the regulations state this figure?

I dont mean why cant we just let it be higher (as it will be protected by an RCD - so in theory (not good practice) just needs to be below 1667Ω)), I mean why 200? Where does this figure come from? Who decided on this figure and more importantly why? What happens at 200Ω that makes it safer than 250 or 300 for example?

Why not have a much lower figure that might actually aid in disconnection times. Is it to do with the logistics of having to add maybe 3 or 4 rods to achieve such figures or other reasons?

Thanks all...

davesparks · May 26, 2014

What you've achieved is getting it closer to a useful value.

Knobhead · May 26, 2014

happyhippydad said:
Sorry to discredit you Tony, but you're wrong again!

Then why did you not say it was part of an EICR first off?

You’re happy with 75Ω are you?

Deleted member 9648 · May 26, 2014

happyhippydad said:
Sorry to discredit you Tony, but you're wrong again! Try the glass half full way of thinking

I had an EICR where the Ra was 256Ω, Obviously wasn't happy with this as I see it as a high figure and assumed it would be a c2, although after reading many threads it seems its a c3. The customer was completely happy to have another rod sunk, It was difficult to find a suitable spot to locate it but I got it down to around 75Ω (approx- cant remember exactly), still not great but better, but it made me think what have I actually achieved??

Tony said:
Then why did you not say it was part of an EICR first off?

You’re happy with 75Ω are you?

Aside from Marvo's point about certain equipment requiring lower earth resistance values than are generally achieved on UK TT systems,which I accept but which generally dont apply to most of the TT installs discussed on here,nobody has ever come up with a satisfactory explanation of what HHP's 75 ohms will do for fault protection that the original 256 ohms wont.
IMO you either get a TN value,or it doesnt really matter if it's 10 ohms or 200 for a small TT install. I would agree with E54 that 7671 needs an overhall regarding TT systems,I believe the recommendation should be for a sub 1 ohm value,or if that is not practical the installation must have an S type back up RCD.

HHD you have met the obligation regarding an EICR with an Ra value of 75 ohms,it meets the requirements of 7671 which is the only purpose of an EICR. Dont take any nonsense *removed*.

HappyHippyDad · May 26, 2014

Tony said:
Then why did you not say it was part of an EICR first off?

You’re happy with 75Ω are you?

Pharrell Williams - Happy (Official Music Video) - YouTube :daisy:

SPARTYKUS · May 26, 2014

happyhippydad said:
Sorry to discredit you Tony, but you're wrong again! Try the glass half full way of thinking

I had an EICR where the Ra was 256Ω, Obviously wasn't happy with this as I see it as a high figure and assumed it would be a c2, although after reading many threads it seems its a c3. The customer was completely happy to have another rod sunk, It was difficult to find a suitable spot to locate it but I got it down to around 75Ω (approx- cant remember exactly), still not great but better, but it made me think what have I actually achieved??

I know what the ground's like round your way....

ipf · May 26, 2014

Never seen a hippy without a few decent spots, usually covered in 'em

Marvo · May 26, 2014

I have a question that's slightly off track but not completely off-topic maybe one or two of the senior guys might remember the answer.

What was the maximum allowable Ra impedance value on a TT earthing arrangement in the 16th or 15th or maybe 14th edition before RCD's were generally available or before there was an option to sell you soul and put all your eggs in one basket by using a 100mA upfront RCD to achieve your disconnection times?

HappyHippyDad · May 27, 2014

Marvo said:
I have a question that's slightly off track but not completely off-topic maybe one or two of the senior guys might remember the answer.

What was the maximum allowable Ra impedance value on a TT earthing arrangement in the 16th or 15th or maybe 14th edition before RCD's were generally available or before there was an option to sell you soul and put all your eggs in one basket by using a 100mA upfront RCD to achieve your disconnection times?

I really hope this gets an answer.. Would be very interesting!

davesparks · May 27, 2014

I've got one for you Marvo:

According to Stubbs the IEE wiring regulations (10th edition I think at that time) require that earth leakage protection is required for a supply up to 100A if the earth continuity conductor is not connected to a town's metallic water main.

It goes on to say the protection is afforded by an earth leakage circuit breaker.

A figure of 40Volts potential rise is mentioned in various entries as a limit to the maximum permissible rise of the earth conductors potential above that of the general mass of earth.

Also the same regulations appear to require that the resistance between any part of the earth continuity conductor and the point of connection to earth must not exceed 1 ohm.
The earth continuity conductor appears to be what we now call CPCs

Engineer54 · May 27, 2014

When i first came into the industry, the accepted minimum TT value was 10 ohms which more often than not was always bettered. But in those days the rods were all extendable 5/8'' - 3/4'' 8 to 10 footers, not the now typical short 1.2m affairs and certainly not those useless 3/8'' twigs they now jokingly sell as earth rods. When i used to help my uncle after school and at weekends, he's method was 2 single rods at suitably spaced positions...

Engineer54 · May 27, 2014

davesparks said:
I've got one for you Marvo:

According to Stubbs the IEE wiring regulations (10th edition I think at that time) require that earth leakage protection is required for a supply up to 100A if the earth continuity conductor is not connected to a town's metallic water main.

It goes on to say the protection is afforded by an earth leakage circuit breaker.

A figure of 40Volts potential rise is mentioned in various entries as a limit to the maximum permissible rise of the earth conductors potential above that of the general mass of earth.

Also the same regulations appear to require that the resistance between any part of the earth continuity conductor and the point of connection to earth must not exceed 1 ohm.
The earth continuity conductor appears to be what we now call CPCs

I'm pretty sure their was no earth leakage protection device available around the time of the 10th edition, certainly not for domestic or average installation use. Even VOELCBs weren't around at that time. About the only place you would find any form of earth leakage protection would be on main switchboards, in the form of Restricted Earth (and the like) protection relays...

Knobhead · May 27, 2014

Sorry E54, I’ve got the same version of Stubbs. Page 410/411 Vol 2

Dave’s bang on with that one.

davesparks · May 27, 2014

[ElectriciansForums.net] TT system - Why less than 200 π ?

Stubbs electrical encyclopaedia, 1938 edition, pages 402 & 403

Knobhead · May 27, 2014

Any advance on pages 401 → 412

PS,
Finally found out where the figure of 1/10,000[SUP]th[/SUP] for maximum leakage current came from. I was taught it as an apprentice and have used it ever since for power systems.

Engineer54 · May 27, 2014

Tony said:
Sorry E54, I’ve got the same version of Stubbs. Page 410/411 Vol 2

Dave’s bang on with that one.

Learn something new every day!! lol!!

Now has anyone here ever seen one of these earth leakage breakers from the middle to late 1930's because i know that i haven't?? So i'm wondering just how generally available they would have been at that time? Not very would be my guess, in fact probably as rare as hens teeth!!

Whether they were included in the IEE reg's of the time, i wouldn't like to say yea or nea now! ...lol!!

Starting work in the 70's for a very large manufacturing company on a site that covered an area of a couple of square miles with 7 factories including a power station/forge and a foundry, the only earth leakage devices i knew of, was limited to main switchboards and main distribution boards in the way of protection relays. Only in the latter part of my time at the company, did i see RCD devices being brought into service, that would be around the same time as circuit breakers and MCB's etc were replacing fuse board installations....