P.M.E. and a Broken Neutral.

Interesting stuff  it seems there's a bit of confusion over who is responsible for mains cut outs  also watched a video from SSE About diverted neutral currents seems its a real problem

IMHO we need to return to proper earthing and cease new PME installations.


In built up areas, 4 core mains with the armouring providing a metallic earth path back to the substation. This armouring to be additionally earthed by local earth rods at each consumer, where possible, and by earth tape at joints in addition.


In rural areas, TT with a requirement that new installations have DUPLICATED RCD/RCBO protection. E.G. each circuit on an RCBO and with a from end 100ma time delayed RCD to give a second line of defence.

It is customary to make some reference, page 44 IET Guidance Note 8. So there are two basic forms of TN-C-S systems: TN-C-S system with PME and TN-C-S system with PMB.

Fig 4.9 shows the PNB earth neutral link can be done at the installation end and I have seen this at main switchboards with a 11KV/400Vtransformer not far away outside. The problem with this is (If there is a problem) with a three phase supply (single phase as well ) if overhead wiring is divided up among a number of dwellings is that on a unbalanced load there is a current flowing in the neutral , (could be large) and this the conductor we are connecting the earth to.

Quarries: Someone said Quarries now come under BS7671 after about 30 40years having there own statutory regulations and having before regulation earth conduction must be at least 50% of line conduction. It is incomprehensible to think that you could apply BS7671 to quarries fixed or portable plant and comply.   


The Types Of Earthing System as showing on the inspection form.

BS 7671 defines four types of earthing system:

a) TN-S

b) TN-C-S

c) TT

d) IT.

Where above apply to quarries where PME is not allowed, I would say PMB, where is the tick box.

Below is where PME is not allowed; (reference from any one of the supply providers)


1 Installations Where A PME Earth Terminal Shall Not Be Offered

NOTE: In certain cases, although parts of an installation may be prohibited from

use of a PME Earth Terminal (e.g. Caravan or Boat) other parts may not (e.g.

permanent buildings at Caravan Park, Camping Park or Marina). This section

shall be read in conjunction with 4.2.2.

PME shall not be offered if this would allow the connection of the PME Earth

Terminal to any metalwork of the following:

a) Caravan. However, see 4.2.2.1 for Caravan Parks/Camping Parks.

b) Fairground, Amusement Park and Circus Temporary Electrical Installations

for Structures, Amusement Devices and Booths.

c) Mobile Home/Residential Park Home if specific conditions apply making it

effectively equivalent to a Caravan. See 4.2.2.1 for Mobile Home Parks.

d) Boat, Houseboat or Boat Mooring Supply. However, see 4.2.2.5 for Marinas

and similar locations.

e) Outdoor Amenity Showers and Outdoor Amenity Facilities.

f) Fuel Filling Station. However, see 4.2.2.6 for Motorway Services Premises

and similar locations adjacent to Fuel Filling Stations. Areas where

dangerous/explosive substances are stored are treated in the same way as a

Fuel Filling Station.

g) Mine Underground Shaft. However, see 4.2.2.8 for Mine permanent

buildings.

h) Quarry Production Area. However, see 4.2.2.9 for Quarry permanent

buildings.

i) Construction Site or Demolition Site. However, see 4.2.2.7 for Construction

Site permanent buildings.

j) Multiple Occupancy Buildings, including flats, maisonettes, bed-sits,

industrial units and commercial premises in Multiple Occupancy Buildings.

See ST:SD5C.

k) Shared communication towers.

l) Impressed Current Cathodic Protection Systems.

m) An AC electrified railway/tramway, including traction supply point

substations

, train stations, signals, level crossings, point heaters and the like.

However, see 4.2.2.13 for LV supplies to DC electrified and non-electrified

railways/tramways, and 4.2.2.13 for details.

n) Mobile phone base stations with antennae on HV towers. See ST:SD6E.

o) Direct supply to a Solar PV System (i.e. solar farm).

Note: Quarries I no longer now electrical  inspect, but still remember the stress.

Please no hostility towards my post

regards

JCM

Where above apply to quarries where PME is not allowed, I would say PMB, where is the tick box.

I think part of the confusion is that not only are there two version of TN-C-S (PME and PNB) there are also two versions of PNB (TN-C-S and TN-S) - i.e. PNB could be either TN-C-S or TN-S depending on relative positions of the Earth connection and the N-PE link.


Some DNOs seem to suggest that PNB should always be treated as if it's PME - not because it necessarily is - but as a safety margin in case the DNO re-arrange things and make it TN-C-S at a later date.


   - Andy.

"


I agree. If possible a different make and also in a slightly different location (mainly "stiction" and damage issues) although usually not practical.

You would need to ensure the 30mA RCDs (RCBOs/RCCBs) auto switch on the N pole too).


AJJ,

nicely put

If this was done you could have RCDs at the substation level, or perhaps more useful something that reported the imbalance current back to the DNO but did not trip anything, allowing developing faults on the network, or customers doing nasty things, be identified early.

A fully TT network would allow the same.

Mike.

You could always watch the PME video on virtual Elex show here https://issuu.com/hamervillemediagroup/docs/iet_virtual_showcase?fr=sOGExYjM1MTc1OTA


You might very well think that my IET chums set me up at the end, however I could not possibly comment!

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Mike.