Basic insulation exposed in electrical outdoor meter box.

It would have to be part of a subsequent amendment ... but if the discussion in the industry never happens, we won't ever see any proposals.

I did try to advise earlier in the discussion, that the concept of preventing access to ordinary persons by limiting access only 'by the use of a key or [a] tool' is a concept that has a long history in electrotechnical standards for both products and installations ... a bit of background that's important to consider if we want to change these words in some standards.

My twopennethworth.... I don't think the intention is to prevent access by ordinary persons - after all access to keys is often driven considerations other than electrical safety (e.g. protecting property from theft or information from unauthorised access) and every Tom, Dick and Harry has a screwdriver. Rather, I suspect, the intention is to prevent accidental or unthinking access - along similar lines to access only being available after a 'deliberate action' we see elsewhere in the regs. If someone takes a screwdriver to an obviously electrical box, on their own head be it, it's more the innocent passers-by with not a clue of the dangers and weren't attempting to carry out any electrical work that deserve the protection.

So I think the problem arises when people expect to be able to open an enclosure and not expect an electrical hazard to result.

So I'd suggest that the 'by the use of a key or tool' bit is fine, but the general concept of an enclosure should be limited to a dedicated electrical enclosure - i.e. something that would only need to be opened to carry out electrical work on what it contains and for no other reason. So a socket or switch or joint box or CU would be fine. But something that people could genuinely expect to open for other purposes - a plumber opening a screwed down bath panel, or an ordinary person opening a meter box to take a meter reading, or a locked cupboard the caretaker also kept his brooms and cleaning materials in, or an entire building that happened to have a lock on every external door, would all be out.

There perhaps could be an alternative option of labelling, to draw attention to the electrical hazard, to cover any corner cases where an electrical enclosure also needs to be opened for other purposes and there's no interlocking or similar protection.

   - Andy.

AJJewsbury said:

So I'd suggest that the 'by the use of a key or tool' bit is fine, but the general concept of an enclosure should be limited to a dedicated electrical enclosure - i.e. something that would only need to be opened to carry out electrical work on what it contains and for no other reason

This concept might be OK for private dwellings, but it is an issue for premises in which the Electricity at Work Regulations applies ... including, perhaps electrical laboratories and workshops.

Personally, I wouldn't object to such a rule for domestic installations, but it would make some provision of BS 7671 more complex.

I would further add, that there's nothing to stop any other enclosure using "meter box" or "lamp post key" in any type of installation ... and such keys are readily available.

but it is an issue for premises in which the Electricity at Work Regulations applies ... including, perhaps electrical laboratories and workshops.

What sort of thing did you have in mind? I was thinking that an entire room could meet the definition of a 'dedicated' enclosure if only electrically skilled/instructed people had access ... but allowing the caretaker to store brooms in there for example would mean that other precautions would be needed.

I would further add, that there's nothing to stop any other enclosure using "meter box" or "lamp post key" in any type of installation ... and such keys are readily available.

Indeed - likewise a screwdriver - hence my comment about the intent being less to reliably prevent access, and more about making it obvious that  electrical hazards lurk behind. A switchroom with "Electrically authorized personnel only" on it is more obvious than a broom cupboard, even if they both had the same kind of lock on it. Most people would think they should be able to read a meter in a meter box without being exposed to additional risk of shock.

   - Andy. 

Also, will this be agreed with the DNO beforehand when the SWA cable is directly connected into the REC isolator.  

There remains the issue of protecting the circuit from the meter to the consumer unit busbars. Regulations 433.3.1 (iii) and 434.3 (iv) allow the designer to utilise the DNO’s main fuse for this purpose, provided the DNO designer agrees that the characteristics of the device are suitable.

For short-circuit protection a 100 amp BS 88-3 (formerly BS 1361) fuse would provide adequate protection provided the meter tails are a minimum of 25 mm² and no longer than 3 m from cut-out to consumer unit.

Also, will this be agreed with the DNO beforehand when the SWA cable is directly connected into the REC isolator.  

There remains the issue of protecting the circuit from the meter to the consumer unit busbars. Regulations 433.3.1 (iii) and 434.3 (iv) allow the designer to utilise the DNO’s main fuse for this purpose, provided the DNO designer agrees that the characteristics of the device are suitable.

For short-circuit protection a 100 amp BS 88-3 (formerly BS 1361) fuse would provide adequate protection provided the meter tails are a minimum of 25 mm² and no longer than 3 m from cut-out to consumer unit.

AMK said:

meter tails are a minimum of 25 mm² and no longer than 3 m from cut-out to consumer unit

Dumb question: is there a quotable calculation of the impedance of such a cable for ensuring sufficient prospective short circuit current to ensure the fuse blows? - I ask as an 'electronics' engineer and having seen lots of questions about the "why 3m" limit

I believe it’s more a rule of thumb precaution due to supply loop impedances as DNOs are under no legal obligation to provide or maintain any particular value of earth loop impedance and during the lifetime of an installation a number of circumstances may alter the earth fault loop impedance.

Philip Oakley said:

Dumb question: is there a quotable calculation of the impedance of such a cable for ensuring sufficient prospective short circuit current to ensure the fuse blows? - I ask as an 'electronics' engineer and having seen lots of questions about the "why 3m" limit

Yes, in a TN system, if you assume the distributor's cutout can provide ADS (there's no guarantee they will agree to that). Table 41.4 of BS 7671 implies 0.38 ohms for a BS 88-3 fuse (a BS 1361 in some service heads would require around 0.35 Ohms taking into account 230 V and C_min of 0.95 in case anyone looks back at the loop impedance tables in 16th Ed) ... something that's probably OK in most TN-C-S installations, but not in some TN installations. The use of an SWA cable brings in the need for ADS (normal "tails" don't require ADS as they are considered equivalent to double or reinforced insulation, something SWA in general can't achieve.)

... which causes a problem in a TT system unless you fit an RCBO or RCCB in the meter cupboard.

In fact, a 300 mA RCBO would work for both TN and TT.

However, the 3 m limit is not for protection against electric shock, but protection against overcurrent ... one of the requirements for not having to provide protection against overcurrent in certain circumstances.

AMK said:

I believe it’s more a rule of thumb precaution due to supply loop impedances as DNOs are under no legal obligation to provide or maintain any particular value of earth loop impedance and during the lifetime of an installation a number of circumstances may alter the earth fault loop impedance

Legal obligation or not, when I reported a slightly high Ze (say 1.05 Ω) to my DNO, they were round like a shot. There was some debate between the operatives as to whether to leave me connected. Within the day, they had dug up the side lawn (using a road breaker 'cos the weather was the same as now) and "PME'd" the supply.