Greenhouse Electrics

[JohnW2]

Too many variables in the equation means there is not, cannot be, one fit all solution to the question.
And then there are too many different way to interpret the often conflicting regulations.

Quite. That's why I was hoping to gain some insight into the sort of thought processes people applied when deciding the best way to deal with such situations (and interpret the regulations).

Kind Regards, John.

 

[JohnW2]

However, I really don't see the point these days.

An old hand many years ago use to put a few amps into the earth wire when measuring the resistance of the earthing. His reasoning was that a short length of thin ( corroded ) wire could still return a low impedance reading but melt if it had to carry a substantial fault current until the fuse wire melted.

You mean he decided the result of his test according to whether or not the earthing conductor melted?! Unless the wire really was on its very last legs or he used a dangerously high voltage for his test, I doubt that he would ever have melted the wire. With 230/240V, he'd be hard pressed to get much more than 5A with most TT rods - and even 0.5mm² of surviving copper (or copper equivalent) would probably tolerate that. If he had such concerns, maybe he should have just measured rthe resistance of the wire?

With RCDs protecting the system the earth wire should never have to carry more than 30 milli-amps so proving the earth wire can carry the fusing current is no longer necessary. Or is it ?

Even if there were no RCDs, I think the exercise would still be futile with most TT installations, since with most such installations there really isn't much hope of getting a high enough current to earth to operate a 6A MCB (or 5A fuse), let alone a protective device of greater rating - so I think the outcome of a test to see if the earth could support OPD disconnection would be a foregone conclusion

Kind Regards, John.

 

[bernardgreen]

You mean he decided the result of his test according to whether or not the earthing conductor melted?!

No He measured the voltage along the wire being measured and with known current R = V / I gave him the resistance. He measured the current using a 1 ohm standard resistor.

He'd be hard pressed to get much more than 5A with most TT rods

I recall he didn't trust ground rods for anything more than an amp. Even less in a drought. Water pipes were his prefered heavy current (fuse blowing) earthing until they went plastic.

 

[JohnW2]

You mean he decided the result of his test according to whether or not the earthing conductor melted?!

No He measured the voltage along the wire being measured and with known current R = V / I gave him the resistance. He measured the current using a 1 ohm standard resistor.

Oh, I see, he just measured the resistance of the wire (as I suggested). Any idea why he felt the need to use 'a few amps' to measure its resistance - did he really think he'd get significantly different answers that way?

He'd be hard pressed to get much more than 5A with most TT rods

I recall he didn't trust ground rods for anything more than an amp. Even less in a drought. Water pipes were his prefered heavy current (fuse blowing) earthing until they went plastic.

That's probably a bit pessimistic (although I'd never knock 'conservatism'); even during periods of 'drought', I don't think I've ever seen mine (usually about 70&#937 get as high as 100Ω. 1A or less would not, of course, satsify current guidelines, since it would imply an impedance greater than 200Ω.

Kind Regards, John.

 

[westie101]

Found this in our COP on customer earthing in reference to PME (TNC-S) systems

Where a service or sub-service is installed to a shippon, greenhouse, milking parlour or similar outbuilding, where persons or animals could be subjected to a difference in potential between say a wet floor and bonded metalwork, it may be impracticable to bond all extraneous conductive parts in that installation to the PME terminal or to include an earthed metal grid in the floor screed. The following steps shall be taken.

(a) The outbuilding wiring shall be connected to an independent earth
electrode, installed outside the influence zone of the PME earthing
system and protected by a RCD.
(b) The product of the rated residual operating current of the RCD and the
associated earth electrode resistance shall not exceed 50V, although
where cattle or similar livestock are present a lower value is preferable.
(c) In order to keep the earthing systems separate, it is essential that any
earth conductor, sheath and armouring of the subservice shall not be
connected to the remote building earth system. Such conductors shall be
double insulated anywhere where they may be exposed within the remote
building. In addition any common metalwork such as piping between the
main and remote buildings shall contain insulating inserts. These inserts
shall be in free air at the point of entry into the remote building, with the
remote building side bonded to the independent earth electrode.
The metalwork on one side of the insert shall be double insulated
anywhere where it comes in such proximity to any metalwork on the
other side of the insert, such that both may be touched simultaneously.
(d) Where the provisions of (c) above apply, a minimum 300mm insert shall be used for pipes of up to 25mm diameter, and proportionally longer
inserts for larger diameter pipes. A longer insert may remove the need
for double insulation.
(e) Pipes and metalwork of outbuildings without an electricity installation
shall be similarly segregated from metalwork connected to the earthing
terminal as described in (c) and (d) above.

 

[JohnW2]

Found this in our COP on customer earthing in reference to PME (TNC-S) systems

Thanks. That's interesting. It therefore seems that, per that Code, my 'preferred approach' (separate, isolated, TT system) is actually the only acceptable approach.

Mind you, as I think was implicit in what ericmark was saying, it begs the question as to what one does if one is in a tightly built-up area and cannot install a TT electrode outside of the influence zone of the PME system.

Kind Regards, John.

 

[bernardgreen]

(c) In order to keep the earthing systems separate, it is essential that any earth conductor, sheath and armouring of the subservice shall not be connected to the remote building earth system. Such conductors shall be double insulated anywhere where they may be exposed within the remote building. In addition any common metalwork such as piping between the main and remote buildings shall contain insulating inserts. These inserts shall be in free air at the point of entry into the remote building, with the remote building side bonded to the independent earth electrode.

This is the "problem" I have in separating a shop from a cottage that were once a single premise, business and home for the shop keeper, but are now separately owned. The supply and meter are in the shop and are TT. The new supply requires total separation of pipe work and earthing, bonds and any other metallic connections between the shop and cottage. otherwise it has to be TT and even then its new ground rod has to be outside the influence of the shop's ground rod.