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

[JD6400]

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 .

 

[Deleted member 26818]

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

Below 10 Ohms is nice, but often we have to just be happy that it's below 100 Ohms.

 

[snowhead]

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!

Can we have a Poll on that

 

[baldsparkies]

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

 

[HandySparks]

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.

 

[HandySparks]

And, to answer the original question, for an installation protected by a 30mA RCD, I'd accept a Zs of 200 ohms on an EICR without any additional comment.

For a new rod, then I'd attempt to comply with the "as low as practicable" note (there's a subjective requirement if ever I saw one!).

Incidentally, I note that Ra includes the resistance of the earthing conductor and the final circuit cpc up to the exposed conductive parts.

In 411.5.3 the regs seem in some confusion between Ra, Zs and "the resistance of the earth electrode", which are, of course all different (although in most cases, not by much).

(Should be subscript 'A'; how do you do subscript?)

 

[HappyHippyDad]

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 .

You're quite right JD, I shall choose my letters more carefully next time.

 

[baldsparkies]

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

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.

That's what happens when the old bugger doesn't put his glasses on.
Many thanks for correcting me kind sir.
Just wish they could come up with some decisive information or even come to a reliable conclusion as to what makes a max value stable and then at what point its not.
I have corrected TT system readings above 400 ohms.
And yet the 30ma rcd's protecting those circuits have, under test, operated within 40ms/200ms no problems.
If un-stability is a science it needs practical clarification so that we have a definitive cut off value that all adhere to with no grey areas.
In the mean time I will continue to go with 100 ohms purely because logic dictates its a safer bet than 200 ohms.
No other reason, and if someone proved conclusively that below 200 is still a safe option then so be it.

 

[D Skelton]

A meaningless poll without a 'depends' option.

I've accepted far higher, I've also refused to accept far lower.

 

[netblindpaul]

You need to refer to BS7430 before you decide, and if you are designing & installing means of earthing, then you must have a copy of this standard to reference.

 

[baldsparkies]

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 ?

 

[davesparks]

How many electrodes and how deep do they go? A higher Ra value from multiple 8' rods properly spaced will be far more stable than a lower value on a single 4' long rod

 

[Engineer54]

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 ?

That's the point there is no set stable value. The only way to determine stability is to test the TT system during different seasons of the year (eg wet and dry seasons) If there is a significant difference in Ra values between the two, then it's not stable. Generally you will achieve a good deal of stability from the depth the electrodes are driven, in decent soil conditions, and why i always advise a minimum of two extendable 5/8'' rods. You'll never in a month of Sunday's ever achieve a stable TT Ra value using a 1.2m thin twig.


You can however quite reasonably have a stable 200, 300, ....500 ohm electrode, which again makes the statement in BS7671 that many still use as a max Ra value, not worth the paper it's written on!!

 

[baldsparkies]

The relevance was often a sore point back in the days when we were completing installations for petrol filling stations.
We worked at the time to the old HS G 41.
I believe BS/EN 60079 is the current standard alongside 3871.
We worked towards a value of sub 20 ohms for TT
Rod or earth plates being employed.
Often the general mass of steel work and even the tanks themselves would bring the values well down once main, and cross bonding were in place.
Now that's when the bonding regime of the 15th really came into its own.
The risks of diverted neutral currents left PME on the shelf.
Guaranteed TN-S was also used, but what was meant by guaranteed was any ones guess.
Always seemed a joke to see all the bonding, cross bonding, earthing arrangements, and precautions taken by the tanker drivers when hooking up.
And then along comes a young lad on his moped, straight into the hazardous zone, with a nice fat couple of thousand volt spark jumping across his dodgy HT lead.

 

[Engineer54]

DNO substations also work to a 20 ohm value, but very few will be anything like 20 ohm, most will indeed be sub 1 ohm, but then again, i've never seen 1.2 metre earth rods being used at any substation they would generally be 5/8'' or 3/4'' extendable 2.4 or 3.2 metre examples and will often be dropped into drilled bore holes surrounded by Bentonite or Marconite. Earth plates are fine the biggest drawbacks to their use is that the ground needs to be excavated first Which generally means that they are often not installed to any great depth, and you can't reliably test them until they have been backfilled and compacted. Same goes for copper tape grids, both systems being far more expensive to implement than installing ground rod electrodes...

Interconnecting different earthing systems say Ufer, underground metal tanks, and made electrodes also needs some thought put into it, as spheres of influence can overlap that can have detrimental effects to the overall Ra values, it's not just an easy case of connecting everything together and hoping for the best...

The bulk fuel tankers connect to storage tanks at filling stations and bulk storage facilities is a precautionary measure to reduce/eliminate any static electricity on the connecting pipework between carriers and static tanks that volatile fuel will be passing through.... I thought it was common practice NOT to fill vehicles at filling stations with the engine still running, i know it used to be....