Streetside metal feeder pillar with PME supply

Open-PEN Detection Devices (OPDDs) are only recognized in BS 7671 for disconnecting the supply to an EV charging point, not other equipment.

Why has identified EPR of the feeder pillar itself as a particular risk, especially if it's installed according to BS 7671 considering the recommendations of ENA ER G12/5,  given that Regulation 114.1 of BS 7671 says:

For a supply given in accordance with the Electricity Safety, Quality and Continuity Regulations, it shall be deemed that the connection with Earth of the neutral of the supply is permanent. Outside England, Scotland, Wales and Northern Ireland, confirmation shall be sought from the distributor that the supply conforms to requirements corresponding to those of the Electricity Safety, Quality and Continuity Regulations (ESQCR), in this respect. Where the ESQCR does not apply, equipment for isolation and switching shall be selected accordingly as specified in Chapters 46 and 53.

Jeff Saunders said:

The bonding conductor from the MET to the feeder pillar is before the O-PEN device, so under PEN fault conditions the outgoing circuits will disconnect but the feeder pillar will rise in potential and will not disconnect from the supply.

This is no different to any other feeder pillar or item of street furniture, or other outdoor equipment or machinery.

The difference with an EV (vehicle itself) is, the vehicle is separated from Earth by rubber tyres, whereas the feeder pillar is generally ground-mounted in itself.

Jeff Saunders said:

Regulation 411.4.2 talks about the recommendation of installing an additional earth electrode on TN systems at the MET. Is this a feasible option to reduce the touch voltage on the feeder pillar under PEN fault conditions, if you could get a low enough resistance on the electrode?

This is not practicable, because of the required earth electrode resistances that you can achieve in most situations. The exception would be a large steel-framed and steel piled building, where that can be used as an electrode.

Jeff Saunders said:

Or are there any other options other than replacing, or installing a GRP feeder pillar in the first place? Or a front-end type O-PEN device?

As above, I would not necessarily recommend a front-end OPDD (and depending on the rating of the supply, it might not be practicable in many installations). A GRP enclosure would do the trick.

So would TT ... BUT only where a simultaneous contact assessment shows there are no issues with simultaneous contact of other street furniture, street lamps, etc., that are not connected to the same TT system.

(Note, the Simultaneous Contact Assessment should also be carried out even if you are using PME supply ... it should be verified with the DNO that their earthing arrangements connected to all simultaneously-accessible exposed-conductive-parts of equipment are connected to the same earthing system - see Regulation 411.3.1.1). 

mapj1 said:

Anything much less than an ohm is likely to be solidly connected.

I'd perhaps not make the statement for all urban arrangements ... the "solid" connection might be via a gas main, which is definitely not ideal. Yes, it will manage the risk  ... at least until the main is replaced with plastic, which is the "but" or "however" part.

The best advice is check with the DNO.

It is unusual to bond modern all-electric lamp posts direct  to gas mains however, so the resistance will be higher if the path is via the nearest buildings on two substations that both share a common gas main.  
I agree the DNO should know, and the gas main replacement folk should be careful what they disconnect, but in reality both might have incomplete records, so to be sure, checking and asking are both a good idea.  As is remeasuring every few years as a matter of course, as things below ground  rot off un-noticed sometimes.

Mike.

mapj1 said:

It is unusual to bond modern all-electric lamp posts direct  to gas mains however,

Existing might be connected to other services through other installations, or accidentally through PILC lead sheath via various means.

mapj1 said:

agree the DNO should know, and the gas main replacement folk should be careful what they disconnect, but in reality both might have incomplete records

Incomplete records is a further point ... in some cases, the DNO may choose to decline to confirm. There are various reasons for this.

mapj1 said:

so to be sure, checking and asking are both a good idea

But that may not yield a simple 'yes' or 'no' answer.

mapj1 said:

As is remeasuring every few years as a matter of course, as things below ground  rot off un-noticed sometimes.

Well ... looking for a value of less than 0.1 Ω isn't generally considered practicable with a 2-lead test set.

The problem is, that 4-wire measurement ("ductor") isn't necessarily safe, or advisable, on parts of earthing systems connected to energized installations (LV or HV).

Great advice again, food for thought, particularly retesting resistances at an EICR to confirm stability.

But, assuming the 2 systems are not common, or are different earthing arrangements entirely and so not compliant with BS7671, there is a new device which is IET01 compliant which detects 21mA of current flowing in the protective conductor of a circuit and disconnects all conductors including protective conductor within 0.8s. Would this provide no-less a degree of protection than compliance with BS7671 for situations where the EV charge point/EV on charge, were the simultaneous contact risk?

Jeff Saunders said:

there is a new device which is IET01 compliant which detects 21mA of current flowing in the protective conductor of a circuit and disconnects all conductors including protective conductor within 0.8s. Would this provide no-less a degree of protection than compliance with BS7671 for situations where the EV charge point/EV on charge, were the simultaneous contact risk?

Let's be very clear, if there is "simultaneous contact", it would have to be the subject of an intended departure from BS 7671, given that "simultaneous contact" of exposed-conductive-parts connected to two earthing systems must not occur where ADS is used (Regulation 411.3.1.1).

According to IET 01, Method of Operation M3 cannot be used as the sole means of protection, even for the intended purpose (protection against EPR from open-PEN faults). This is, quite simply, because someone has to be actually receiving a shock for the current to be flowing that the device detects - ADS doesn't work that way, because the fault is to an exposed-conductive-part, using that path to disconnect.

Therefore, whilst it is true that an OPDD with an "M3" method of protection might operate if someone is getting a shock in 'simultaneous contact' situations, BS 7671 has a provision that prevents the shock in the first place. It would therefore not be possible to declare an intended departure stating "no less safe" (Regulation 120.3) by virtue of the presence of the "M3" device, because not getting the shock in the first place (conforming to Regulation 411.3.1.1) is clearly much safer than hoping the device will operate before exposure to the shock causes injury or death in a particular given set of circumstances.

Thank you for that clarification Graham 

I'd agree that the whole point of ADS is that it trips and removes the danger before someone touches it. 

"Someone has to be actually receiving a shock for the current to be flowing that the device detects"

And that would be a red-line in regard of 'this does not meet the standard of BS7671 or similar' , and is the basis of the standard approaches of bond solidly or separate widely  for indoor installations with more than one earthing system.

But, when discussing metal parts planted into terra-firma, I'm far less sure that it is actually true as the electrode-like behaviour means there is a conducting path of sorts, probably enough to operate something of RCD level of sensitivity,  before the person touches it so a voltage offset could be detected.

Not saying its a great idea mind, just pointing out that many of the normal assumptions don't hold properly once you go outside.

Mike

mapj1 said:

But, when discussing metal parts planted into terra-firma, I'm far less sure that it is actually true as the electrode-like behaviour means there is a conducting path of sorts, probably enough to operate something of RCD level of sensitivity,  before the person touches it so a voltage offset could be detected.

I don't think we are talking about "metal parts connected to the ground" ... the simultaneous contact issue revolves around the requirement that simultaneously accessible exposed-conductive-parts are connected to the same earthing system.

Yes I agree with that - but when things are on separated earths, indoors, they are potentially very separated, and can support a dangerous difference between the two systems that would be a shock voltage, and that may be there without a detectable current flow, until some unfortunate grabs one of each.

In that case there  is no possibility of any pre-emptive ADS cutting power before anyone touches it , only disconnection within the half heartbeat that we require once a shock is flowing.

When bits of metal are actually stuck into the same wet planet and are connected by separate CPCs to different earthing systems, there is an additional rather poorly defined leakage path. 
So, before anyone touches anything,  enough current probably (and this uncertainty is what I don't like... ) flows, before a dangerous touch voltage is reached,  that some form of pre-emptive ADS or warning would possible in principle , based on detecting that leakage current though the terra-firma.
Its not done, not recognized, and a nasty ill-defined thing to rely on, but it is something to be aware of.
Mike.

which detects 21mA of current flowing in the protective conductor of a circuit and disconnects all conductors

if you stick such a device in a system with PME earths and downstream bits and bobs in contact with the general mass of the Earth, I suspect you'll end up with unwanted tripping left right and centre.

As PME earth is also the system N, ordinary voltage drop means it can wander about quite a few volts from true Earth potential in perfectly normal conditions. For instance, say 5V difference and a bonded part acting like a 200Ω electrode, you'd expect 25mA to flow... (and real number may well be less favourable). Not a problem with a EV sat on rubber tyres, but for more general application it might not be so simple.

   - Andy.

which detects 21mA of current flowing in the protective conductor of a circuit and disconnects all conductors

if you stick such a device in a system with PME earths and downstream bits and bobs in contact with the general mass of the Earth, I suspect you'll end up with unwanted tripping left right and centre.

As PME earth is also the system N, ordinary voltage drop means it can wander about quite a few volts from true Earth potential in perfectly normal conditions. For instance, say 5V difference and a bonded part acting like a 200Ω electrode, you'd expect 25mA to flow... (and real number may well be less favourable). Not a problem with a EV sat on rubber tyres, but for more general application it might not be so simple.

   - Andy.