TT/PME Bonding of metal cabinet that houses the DNO Cutout (PME) and Private RCD (TT)

Steve UKBC:

(originally)

So we are left with a metal cabinet where the PME supply switches over to a TT.


(later)

Id prefer not to take the PME earth to the farm buildings and keep the TT/PME switch away from the area and in the cabinet in the field adjacent to the TX pole, as you cannot know the future use or these farm buildings, there is currently no livestock but who's to say this does not change in the near future.

Any outer damage of the feeder cable if the PME earth were to be brought to the farm building (i.e exposed SWA carrying the PME earth) would introduce the PME earth potentially closer to the farm building earth electrodes would it not?

I don't think it is at all correct to say that the PME supply switches over to TT. The distributor's earth is simply sitting there unused.


If DB1 is in a farm building which has potential for housing livestock, I can understand the reluctance to take the PME supply to it, but as things stand you have exposed conducting parts of two earthing systems which are very obviously simultaneously accessible contrary to 411.3.1.1 On reflection, I may upgrade to C1!


Get rid of that shoelace of an earthing conductor between the service head and "MET" and preferably, bung in another earth rod near the origin.

Chris Pearson:

Steve UKBC:

(originally)

So we are left with a metal cabinet where the PME supply switches over to a TT.


(later)

Id prefer not to take the PME earth to the farm buildings and keep the TT/PME switch away from the area and in the cabinet in the field adjacent to the TX pole, as you cannot know the future use or these farm buildings, there is currently no livestock but who's to say this does not change in the near future.

Any outer damage of the feeder cable if the PME earth were to be brought to the farm building (i.e exposed SWA carrying the PME earth) would introduce the PME earth potentially closer to the farm building earth electrodes would it not?

I don't think it is at all correct to say that the PME supply switches over to TT. The distributor's earth is simply sitting there unused.


If DB1 is in a farm building which has potential for housing livestock, I can understand the reluctance to take the PME supply to it, but as things stand you have exposed conducting parts of two earthing systems which are very obviously simultaneously accessible contrary to 411.3.1.1 On reflection, I may upgrade to C1!


Get rid of that shoelace of an earthing conductor between the service head and "MET" and preferably, bung in another earth rod near the origin.

Wording aside.

I agree the earthing conductor from the service head shall be removed. but not shoving a earth rod in as i suspect the cabinet is very close to the TX earth electrode/mat.


We may also still have 2 earthing systems present so to speak, with the potential that the TX/supply earth mat/electrode is close or connected to the cabinet plinth steel work/foundation and so in effect the cabinet is at the same potential as the PME earth/neutral and the metal enclosed fuse switch is connected to the TT earth via DB1 (still in effect two earth systems), this is why i suggest replacing the potentially faulty fused switch with a plastic enclosure (RCD, MCB or fuses etc.). if I'm way of the mark here please say?

Sparkingchip:

Hot tubs, are we going to call them another special location?


You would not dissuade me from ignoring the DNO earth terminal and making the whole installation TT and bonding the cabinet to the earth bar in the main fused switch. 

what are you effectively trying to say here in your second statement ?

Steve UKBC:

Chris Pearson:

Steve UKBC:

(originally)

So we are left with a metal cabinet where the PME supply switches over to a TT.


(later)

Id prefer not to take the PME earth to the farm buildings and keep the TT/PME switch away from the area and in the cabinet in the field adjacent to the TX pole, as you cannot know the future use or these farm buildings, there is currently no livestock but who's to say this does not change in the near future.

Any outer damage of the feeder cable if the PME earth were to be brought to the farm building (i.e exposed SWA carrying the PME earth) would introduce the PME earth potentially closer to the farm building earth electrodes would it not?

I don't think it is at all correct to say that the PME supply switches over to TT. The distributor's earth is simply sitting there unused.


If DB1 is in a farm building which has potential for housing livestock, I can understand the reluctance to take the PME supply to it, but as things stand you have exposed conducting parts of two earthing systems which are very obviously simultaneously accessible contrary to 411.3.1.1 On reflection, I may upgrade to C1!


Get rid of that shoelace of an earthing conductor between the service head and "MET" and preferably, bung in another earth rod near the origin.

Wording aside.

I agree the earthing conductor from the service head shall be removed. but not shoving a earth rod in as i suspect the cabinet is very close to the TX earth electrode/mat.


We may also still have 2 earthing systems present so to speak, with the potential that the TX/supply earth mat/electrode is close or connected to the cabinet plinth steel work/foundation and so in effect the cabinet is at the same potential as the PME earth/neutral and the metal enclosed fuse switch is connected to the TT earth via DB1 (still in effect two earth systems), this is why i suggest replacing the potentially faulty fused switch with a plastic enclosure (RCD, MCB or fuses etc.). if I'm way of the mark here please say?

Well, I'm not sure that you can get a plastic switch fuse, but I stand to be corrected. What I don't understand is why you cannot protect the distribution cable with the suppliers earth. I accept that if the HV earth is nearby, a rod by the cabinet may not be appropriate, but I repeat my questions (1) what is Ze? (Presumably tiny.) (2) What is R1+R2 of the distribution circuit? (3) What fuses are in the switch fuse?

Chris Pearson:

Steve UKBC:

Chris Pearson:

Steve UKBC:

(originally)

So we are left with a metal cabinet where the PME supply switches over to a TT.


(later)

Id prefer not to take the PME earth to the farm buildings and keep the TT/PME switch away from the area and in the cabinet in the field adjacent to the TX pole, as you cannot know the future use or these farm buildings, there is currently no livestock but who's to say this does not change in the near future.

Any outer damage of the feeder cable if the PME earth were to be brought to the farm building (i.e exposed SWA carrying the PME earth) would introduce the PME earth potentially closer to the farm building earth electrodes would it not?

I don't think it is at all correct to say that the PME supply switches over to TT. The distributor's earth is simply sitting there unused.


If DB1 is in a farm building which has potential for housing livestock, I can understand the reluctance to take the PME supply to it, but as things stand you have exposed conducting parts of two earthing systems which are very obviously simultaneously accessible contrary to 411.3.1.1 On reflection, I may upgrade to C1!


Get rid of that shoelace of an earthing conductor between the service head and "MET" and preferably, bung in another earth rod near the origin.

Wording aside.

I agree the earthing conductor from the service head shall be removed. but not shoving a earth rod in as i suspect the cabinet is very close to the TX earth electrode/mat.


We may also still have 2 earthing systems present so to speak, with the potential that the TX/supply earth mat/electrode is close or connected to the cabinet plinth steel work/foundation and so in effect the cabinet is at the same potential as the PME earth/neutral and the metal enclosed fuse switch is connected to the TT earth via DB1 (still in effect two earth systems), this is why i suggest replacing the potentially faulty fused switch with a plastic enclosure (RCD, MCB or fuses etc.). if I'm way of the mark here please say?

Well, I'm not sure that you can get a plastic switch fuse, but I stand to be corrected. What I don't understand is why you cannot protect the distribution cable with the suppliers earth. I accept that if the HV earth is nearby, a rod by the cabinet may not be appropriate, but I repeat my questions (1) what is Ze? (Presumably tiny.) (2) What is R1+R2 of the distribution circuit? (3) What fuses are in the switch fuse?

I said a plastic enclosure and not a plastic switch fuse, the plastic enclosure of which would house a din rail with RCD and MCB, the SWA gland (feeder cable would be shrouded and a warning label attached locally warning of a TT earth present.

AJJewsbury:

RCD in the DNO cabinet would be the better solution since the clearance time for a earth fault may be above 5s

Provided everything is OK for voltage drop and R2 isn't massively larger than R1, I would be surprised if you couldn't get 5s disconnection times from the usual overcurrent protective devices.

  - Andy.

Come to that what is your voltage drop like? 100m is quite long for a LV submain and your diagram seems to suggest it's just 35mm² - on what's presumably something close to a 100A/phase system? While the tabulated v.d. of 1.1mV/A/m will be an over estimate as you'll be running the cable a bit cooler than 70 degrees there doesn't feel to be much scope for achieving the usual 3% overall for downstream lighting circuits - even an overall 5% to the end of final circuits looks rather tight (even if you claim the extra 0.005%/m beyond 100m).


   - Andy.