I presume your concern is lost neutral, and the general advice not to use PME in caravans and similar ?

It may not be what you would do if starting the design again, but it has been working for some years ?

Unless there have been an incident causing a review, or the LV supply is especially vulnerable - overhead singles or rotting consac, I'd suggest do nothing without good reason. For example being landfill, is  it the sort of place one should drive earth rods, or is there a risk to buried plumbing and so on ? It probably is not so bad as a site I recall being on where we decided not to dig or drive rods due to the risk of buried explosive ordnance, but even so there are no prizes for hitting a pipe full of methane.
Unless heavily used, the containers are not much more or a risk to passers by than the lamp posts and there are plenty of those on PME supplies.

In terms of general safety on a lightly used site, things like working RCDs maybe  more worthwhile.

Bonding is a trickly one and may not be particulary practical to apply the BS advice if the main supply is massive compared to the supply to the cabins. In that sense an approach more similar to the street lights may be more sensible - I  presume they are in 4 or 6mm?

Mike.

Hi, what will the site pumps be used for ? Will the site be considered hazardous? (Like a petrol station) If yes, PME may not work well there, as it may cause fire or explosion risks due to neutral currents or voltage rise. You should also make sure there is a qualified person who can manage the use and connection of these units to PME.

A site with 300mm2 supply is likely to be straight to the DNO transformer? No connections to other customers along the route? If so, although not impossible, loss of PEN would be improbable.

unless specifically required to consider open pen conditions, why can’t we simply refer to regulation 114.1 which, in my view, allows the designer to consider that the neutral of the supply provided by the DNO is permanently connected to earth. For that to happen the supply neutral can’t be open.

Thanks for the responses. Thee pumps are borehole pumps for pumping leechate which is liquid which accumulates in the waste. The pumps are atex pumps and there are also level transducers in the wells connected to the control panels via IS barriers. The control panels have to be kept more than 2.2 meters from the wells as this is atex zone 2. Gas is extracted from the wells to the gas plant which is a separate installation. Other than the area around the wells there are no other atex zones in the area of the electrical installation. 

The bonding in the cabins is in a mixture of sizes: 6 to 16 mm2 but then reliant on cable armour in some instances for connection back to the Installation MET. The smallest armour relied on for this is on 25mm 4 core, the smallest copper core relied on for this is 10mm2. 

Thanks for the replies.

The bonding connections locally in the containers range between 6 and 16mm. The connection back to the MET relies in some case on cable armour, the smallest being 16mm 4 core. The smallest core relied on is 10mm.

The pumps are atex borehole pumps for leechate, which is liquid which accumulates in the waste. The area around these leechate wells is atex zone 2. There are also transducers in the wells connected into the control panel via IS barriers. The control panels are kept more than 2.2 metres from the wells so that they are not in an atex zone. 

The risk with a former landfill site is methane, so yes there will be a risk of explosions. As AMK says, treat as if it were a petrol filling station or gas pumping station. The pumps are probably for for water though, with contaminated water being collected in an underground tank.

regards, burn

Damo,

Ok, you indicate the pumps are EX certified so somebody has decided there is a potentially flammable atmosphere, the transducers are obviously connected to a properly designed and calculated intrinsically safe circuit.

Now in the appropriate standard 60079-14 its quite clearly states that TNC-S and TNC Type earthing should not be used for Haz area circuits, so ideally TT or you can guarantee a dedicated TN-S system. Isolation and protective device needs to break all live conductors, so that also needs to be considered for protective devices and points of isolation for those Haz area circuits.

DSEAR:2002 indicates a area classification ndrawing should be produced so that will help identify the haz areas and ideally avoid putting anything electrical or sources of ignition in them.

Cheers GTB 

Thanks for the advice.

First I should correct something in my previous post. The borehole pumps may not have atex motors but the motors are considered to be outside the hazardous area since they are under the level of the leachate (the pump inlet is above the motor so the motor is always submerged). Atex pumps are always used in ko pots which are points on the gas line where condensate is extracted but there are none on either of these installations. So within the zones we have a pump in the well in the area above the liquid which is bolted to the motor which has an earthed casing. The area above the liquid would usually be zone 1. The H07 cable to the motor passes through zone 1 in the well and zone 2 in the area immediately outside the well to a control panel outside the hazardous area.

Is the bit that says TNCS earthing cannot be used clause 6.3.2 of been 60079-14? I find the wording of this confusing. It talks about transition from tn-c to tn-s. A system that “transitions” from tns to tnc sound like tncs. What does that mean? The way the clause is phrased it only seems concerned with N-PE connection in the hazardous area.

“60079-14 - 6.3.2 TN type of system earthing If a type of system earthing TN is used, it shall be type TN-S (with separate neutral N and protective conductor PE) in the hazardous area, i.e. the neutral and the protective conductor shall not be connected together, or combined in a single conductor, in the hazardous area. At any point of transition from TN-C to TN-S, the protective conductor shall be connected to the equipotential bonding system in the non-hazardous area.”

It is also not obvious to me why a tncs supply from eg a private transformer to a single installation would pose a risk.

Another aspect to consider is that this has come up as an issue on eicr. (With an agreed limitation to test only to the control panels). To what extent can compliance with bsenb60079 be taken into account for the EICR? Of course I can raise the issue separately if that is more appropriate.

Damo said:

It is also not obvious to me why a tncs supply from eg a private transformer to a single installation would pose a risk.

In any type of TN-C or TN-C-S system, load Neutral current travels through Earth (as in the general mass of Earth) as well as Neutral ... unless, that is, you have a perfectly balanced three-phase supply.

When you apply bonding in TN-C-S systems, load current can also pass through protective conductors.

Basically, where BS EN 60079-14 applies, don't want protective conductors (and in particular any protective bonding system) in the hazardous area to pass varying amounts of load current.

This is why PME is generally prohibited as a supply for petrol filling stations (PFS), and where the PFS has a TN supply, the APEA Blue Book recommends using a TT earthing arrangement, at least for the hazardous area equipment and associated supplies.

gkenyon said:

When you apply bonding in TN-C-S systems, load current can also pass through protective conductors.

Please correct me if I am wrong, but surely not in final circuits except under fault conditions or only in the milliohm range if there is any "leakage"? So if only final circuits are in the zone, all should be well.

If I am correct, it may be that the transition from TN-C to TN-S which Damo mentions could take place in the final DB.