Does everything need to be a poll?
We'll be having designs put to a poll soon!
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
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 .
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.
A meaningless poll without a 'depends' option.
I've accepted far higher, I've also refused to accept far lower.
Including the depends option, would be interested to know your reasoning behind acceptance vs refusal.
Its the one area where over the years I have never seen agreement on what determines a stable value.
At what point does one say yay or nay ?