Domestic Installs Metal CU's also = Metal PV Isolators

[Worcester]

Thanks for the link BruceB

This is the IMPORTANT bit from the BS:

Additional definition
3.1.104 Customer distribution board CDB
An integrated assembly, for the control and distribution of electrical energy, principally in a household or similar premise, incorporating manual means of double-pole isolation on the incoming circuit(s), with polarity observed throughout. They are designed for use exclusively with specific protective devices on the outgoing circuits, and type-tested for use when energized through the specified 100 amp fuse. NOTE Generally known in the UK as a Consumer unit.

So I think that both the IET and NICEIC guys according to the LETTER of the regulations are WRONG when relating to rotary isolators (the important words from above are "and distribution" ); if putting in a 'garage' / 'shower' or similar small unit with a main switch and at least one MCB, then it needs to be in a non-combustible enclosure.

If we consider the SPIRIT of the regulations, then see here for the reason for the introduction of non-combustible CU's: (Consumer units - IET Electrical)

Reason for the new regulation

Regulation 421.1.201 was introduced because of reports from fire investigators of a recent trend of increasing numbers of fires involving consumer units having a moulded thermoplastic enclosure

The cause of the fires investigated was almost invariably found to be resistance heating as a result of poor electrical connections due to poor workmanship or lack of maintenance. Examples of such poor workmanship are thought to include:

- failure to tighten neutral conductor connections to the same torque as line conductor connections due to a misconception that neutral conductors carry less current;
-inadvertently taking cable insulation inside the terminal at a connection, resulting in the securing screw or the clamp of the terminal not making proper contact with the conductor;
- failure to check factory installed connections for tightness where required to do so by the manufacturer; and
- tightening connections to an incorrect torque and/or with inappropriate tools.

The crucial importance of checking that all electrical connections are properly made, and of taking remedial action where necessary, should never be forgotten.

Whilst the main cause of fire within plastic consumer unit enclosures is without doubt poor workmanship, other potential causes of fire within plastic consumer unit enclosures are thought to include:

- the type of internal components, such as where terminations have only one securing screw or are of the 'cage clamp' type); and
-where increased heat transfer to the enclosure is caused by the close proximity to an internal item, such as a neutral bar or a conductor terminated at the neutral bar and protruding above it.

Intent of the new regulation

The intent of Regulation 421.1.201 is considered to be, as far as is reasonably practicable, to contain any fire within the enclosure or cabinet and to minimise the escape of flames.

Then on the basis that the same poor workmanship could occur on a a rotary isolator, double pole MCB or incoming RCD you can see why IET / NICEIC will send out the message that they wish it to be in a non-combustible enclosure

If you've never seen it, then you should seee the sparks and flames you can get when a short circuited DC cable (for Isc testing) on a small domestic installation is pulled apart - it's just like an arc welder.

HOWEVER, (and this is where the argument falls down) I would hope that MCS registered solar PV installers, the installation being of a specialist nature are particularly aware of the dangers of the above poor workmanship issues and so take particular care to ensure those errors don't occur. Unfortunately we have took often come across badly installed isolators.

 

[TedM]

Gordon, I think the BEAMA document I linked to earlier confirms the "and distribution" point you make and with which I agree, when it lists (albeit non-exhaustively) the "similar assemblies" that it says the new reg applies to.

But even if you want to comply with the spirit of the new regs and install a metal enclosed isolator then where are you going to get one from that is also stated to comply with BS EN 61439-3 by the manufacturer?

 

[TedM]

Now I have had time to read through the copy of EN 61439-3 that Bruce linked to, I would say that it is logically impossible for a simple isolator to pass all the tests specified in the standard for the simple reason that some tests (10.11.5.7.2 and 10.11.5.7.4) require testing of 'final circuit protective device(s)' which, of course, a simple isolator doesn't have.

Any manufacturer who states that their isolator complies with BS EN 61439-3 is not going to be telling the whole truth.

 

[Gavin A]

Maybe someone could draft a letter to IET on this that we can all sign, and invite them to respond and be questioned on here?

 

[SibertSolar]

Good idea

 

[Gavin A]

I weren't volunteering btw, up to my eyeballs

 

[SibertSolar]

Haha...I nominate Gavin!

 

[TedM]

I'll stick a post on the IET forum.

 

[TedM]

IET Forums - PV and BS7671 Amd 3

I understand that PV installers have been receiving conflicting advice from different bodies (IET, NICEIC, NAPIT) regarding the applicability of BS7671 Amd 3 421.1.201 and metal enclosures for AC and DC DP isolation switches, which are a standard part of any domestic PV system.

Some are being told that they must install isolation switches with only metal enclosures and others that it is OK to continue to use the normal plastic housed ones.

The BEAMA guidance document does not specifically list PV DP isolation switches as being 'in scope' in their (non-exhaustive) list of "similar assemblies".

As the new regulation has two parts - namely compliance with BS EN 61439-3 and the requirement for a metal enclosure - I have briefly looked at BS EN 61439-3 and it appears that some of the tests required for manufacturers to complete in order to show compliance with this standard can only be applied to assemblies that contain "final circuit protective device(s)" which are obviously absent from a simple DP switch.

There is also a definition in the standard that assemblies have a rated voltage no higher than 300V AC, which would appear to preclude 1000V DC units.

It would therefore appear to be logically impossible for any manufacturer to state that their isolator switch assemblies are BS EN 61439-3 compliant.

And it would then appear that it would be impossible for any PV installer to obtain any isolation switches that can comply with 421.1.201 - whether they have metal enclosures or not.

My interpretation is therefore that DP isolation switches as used in PV installations are, in fact, outside the scope of 421.1.201 and that metal enclosures are not required.

Can anyone provide any clarification on this matter?

Thanks,

Ted

 

[BruceB]

I have had a reply from the IET moderators saying they have drawn Richard Townsend's attention to the thread on the IET website so hopefully we willl soon understand his thinking.

 

[Gavin A]

cheers ted. I'd also post up the NAPIT response as well, to give IET something to chew on.

 

[Gavin A]

ps as this only takes effect from January, it's looking likely to be a bit of a moot point unfortunately

 

[TedM]

IET Response:

The new clause 421.1.201 in BS 7671:2008 (2015) is still being bottomed out by industry, before the caveat requirement takes effect in January 2016. Having spoken to national committee members/Panel chairman, I feel, at this time, that a rotary isolator could be enclosed in a plastic housing, taking account of any external influences. This will be confirmed with formal guidance in due course.

-------------------------
Richard Townsend
Senior Engineer
Standards and Compliance
THE IET

 

[Gavin A]

nice one. Occasionally sense does prevail.