Discuss Would you accept a Ze of 199Ω on a TT Installation which also has an RCD. in the Industrial Electrician Talk area at ElectriciansForums.net

Would you accept a Ze of 199Ω on a TT Installation which also has an RCD?

  • Yes

    Votes: 8 27.6%
  • No

    Votes: 21 72.4%

  • Total voters
    29
Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

HappyHippyDad

-
Esteemed
Arms
Reaction score
5,578
I realise many threads have been discussed around the (maximum) figure of 200Ω for external earth fault loop impedance (as given by BS7671) on a TT, however I'd be interested to see what proportion would and wouldn't accept this figure.

Hopefully we wont get into semantics of was it a dry day, wet day etc. Its just a straight forward simple poll seeing how many would accept the figure given by BS7671.

This will be an anonymous poll.

EDIT : I SHOULD HAVE SAID :

"WOULD YOU ACCEPT A Ze OF 199 Ω ON A tt INSTALLATION WHICH ALSO HAS RCD PROTECTION"
 
Last edited:
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

Should the option on the poll be below 200 ohms?

Yes.. bugger!

Can You change it mods?

I should have put 'Would you accept a Ze = 199Ω on a TT Installation which also has 30mA RCD protection'.

Hopefully you'll all see this and vote accordingly :smile5:
 
Last edited:
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

Does everything need to be a poll?

We'll be having designs put to a poll soon!
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

200 no


199 yes.

unless you are with the NICEIC and then its

100 no


99 yes

:rant::rant::rant:
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

Does everything need to be a poll?

We'll be having designs put to a poll soon!
Like any other thread, you have the option to ignore it if it doesn't interest you.

If it interests nobody it'll soon drop off the bottom of the page....
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

And I'd like to think that all TT systems will have an RCD, multiple RCD's in fact. Even if they meet TN readings I'd still be inclined to have an upfront Type S RCD...
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

Just don't show it to Tony Cable.
He will only frown at you for not listening to those who are above and beyond BS 7671.
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

Yes.. bugger!

Can You change it mods?

I should have put 'Would you accept Ze = 199Ω on a TT Installation which also has 30mA RCD protection'.

Hopefully you'll all see this and vote accordingly :smile5:

Do you mean Ra
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

let's put this stupid 200Ω myth to bed once and for all. it is NOT a max. figure in BS7671, which clearly states that a value above 200Ω is liable to be unstable. there is no mention of a max. figure in any BS7671.

for the record, i'd not accept it and i can't vote in the poll as there is a forum glitch. paul m is looking into it.
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

let's put this stupid 200Ω myth to bed once and for all. it is NOT a max. figure in BS7671, which clearly states that a value above 200Ω is liable to be unstable. there is no mention of a max. figure in any BS7671.

for the record, i'd not accept it and i can't vote in the poll as there is a forum glitch. paul m is looking into it.
Tel why is your text getting so small. (PS I've bought the bloody bonneville!!!!!)
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

I would want to see reading of less than 10, so keep swinging that hammer!!
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

Granted HHD , but you could have just as easily referred to it as Isc for that matter then ! After all terminology is quite important in our industry .
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

let's put this stupid 200Ω myth to bed once and for all. it is NOT a max. figure in BS7671, which clearly states that a value above 200Ω is liable to be unstable. there is no mention of a max. figure in any BS7671.

for the record, i'd not accept it and i can't vote in the poll as there is a forum glitch. paul m is looking into it.

Problem is its open to interpretation, to be fair it implies a 200 ohm ceiling, and that's where confusion can come into play.
Why state instability at 200 ohms if 100 ohms is going to be a cut off point.
They need to clarify and that's been the case for far to long, like I say jokers.

As a point of reference (for what its worth)
Table 45.1
Regulation 411.5.3
Note 2,
The resistance of the installation earth electrode should be as low as practicable.
A value exceeding 200 ohms may not be stable.
Refer to Regulation 542.2.2
 
Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

Problem is its open to interpretation, to be fair it implies a 200 ohm ceiling, and that's where confusion can come into play.
Why state instability at 200 ohms if 100 ohms is going to be a cut off point.
They need to clarify and that's been the case for far to long, like I say jokers.

As a point of reference (for what its worth)
Table 45.1
Regulation 411.5.3
Note 2,
The resistance of the installation earth electrode should be as low as practicable.
A value exceeding 200 ohms may not be stable.
Refer to Regulation 542.2.2

You missed the actual table 41.5 (not 45.1), which says:
Maximum earth fault loop impedance (Zs) for non-delayed RCDs to BS... .... for U0 of 230V...

Rated residual operating current (mA)__________ 30 / 100 / 300 / 500
Maximum earth fault loop impedance Zs (ohms) 1667 / 500 / 167 / 100



So, an installation protected by a 30mA RCD with a circuit Zs of 1667 ohms would comply with the regs, providing that you could show that this was the maximum Zs under all conditions (and that it was not practicable to reduce it).

Not saying that this is 'right' or good practise.

My interpretation of the 200 ohm figure is that if a Zs of 200 ohms or less is measured, then the figure is unlikely to degrade enough to cause the required Zs value to be exceeded (with change of ground conditions). It's odd, however, that they don't give a lower 'may be unstable' figure for installations with higher rated RCDs. After all, even a rod with a low Ra will still vary, although by a lesser amount.
 

Reply to Would you accept a Ze of 199Ω on a TT Installation which also has an RCD. in the Industrial Electrician Talk area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock