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. 

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.

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.

It rather depends what the final circuit CPC is connected to at the load end. A self contained class I item like a car on rubber tyres, I agree with you, under non-fault conditions, there is no path for current along the CPC to get back, and only current flowing originates  from the live supply to that load.

But things with plumbing, or other metalwork stuck in the ground - including cameras mounted on some designs of metal fence post, antenna masts and lamp-posts and so on, in effect have another electrode stuck in the ground at the load point.

Now the regs do not make a clear distinction, but there is a huge difference in the potential fault  current that can flow in the CPC of something that metallically bridges to another earthing system - like a water main shared with the house over the road, and one that only connects through several ohms of terra-firma - such as a pipe to a private tank in the yard that goes no further.

But in any case once you have two electrodes, or connection to another earthing system,  however that occurs,  there is nothing to prevent currents that are nothing to do with your installation, popping up one electrode, through your CPC and off down another, using your  CPC and wiring as a low impedance short cut to parallel up with an otherwise quite resistive chunk of mud.

The problem is also that these currents remain even when the intended load is switched off.

So the safest thing is to avoid there  being such voltage slopes across the ground in the first place, and to do that one has to be out of the near field of any PME earthed hardware.

In the case of the OP with PME earthed street lights etc nearby , that is likely to be impossible to avoid totally, and the question is how good is good enough

ATEX is a significant complication..

Mike.

Chris Pearson said:

not in final circuits

Even in TN-C-S, final circuit load current still has to flow in the TN-C portion of the system.

The issue in PFS, is that they don't want any current flowing through the ground at all ... because if it flows through the bonding grid that can be dangerous.

It does depend geographically how the TN-C bits are arranged, so if it's "linear down a path" the current flowing through the ground can be managed more easily ... but in reality, in a lot of installations, bits of TN-C go all over a site, so ground currents can still occur by shortest path through Earth.

gkenyon said:

Even in TN-C-S, final circuit load current still has to flow in the TN-C portion of the system.

Yes, I get that.

As you say, it probably is all in the geography. There must be plenty of PFSs that have adjacent street lamps and dwellings with metallic services on a PME supply

Chris Pearson said:

As you say, it probably is all in the geography. There must be plenty of PFSs that have adjacent street lamps and dwellings with metallic services on a PME supply

Well, I've seen this in ["private"] installations and had to take a view ... and make recommendations to remedy the issues that can result - and nothing to do with explosive atmospheres.