Todays work

[Sidewinder]

Thanks M, very good post mate, I'll look through the bible later myself.

Anyone else got any regs that may be relevant.

BTW M, no tin hat needed mate, medals more like.

 

[Sidewinder]

532.1 (i) - condition 2

Steps,

This only relates to my beloved BE2 mate sorry, NOT standard domestic TT!

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Ok here we go (why am I doing this..... feels like I'm painting a target on my forehead :^O ) this could be a long one so stay awake....Hopefuly this will point some new peeps to the industry in a direction that they can then decide & interpret the regs for themselves... So a Domestic TT system first thing lets take a look at

411.5

411.5.2 One or more of the following types of protective device shall be used, the former being preferred:

(i) An RCD

(ii) An overcurrent protective device

Now my take on that is an RCBO fits the bill to cover both (i) & (ii). no mention of double pole requirements, so again I would be happy to use either single or double pole devices.

I agree M, & I have cross referenced all of the regs and all nested cross references and I cannot find it written that double pole RCD devices are used under "normal" conditions.

Next is 537.1

This section provides requirements for:

(i) non-automatic local and remote isolation and switching measures for the prevention or removal of dangers associated with electrical installations or electrically-powered equipment and machines, and

(ii) functional switching and control.

So here I am reading the regulation as, the mainswitch can give the provention & removal of dangers by means of isolating both line & neutral of the supply. But no mention of single or double pole requirements (for that we need to read on)...as a side note for those at college or just starting out Table 53.2 gives excelent guidance of switches, their BS numbers & a yes/no guide to their uses for isolation,emergency switching & functional switching.

Have you added in the Corrigendum from 07/2008 into your book with regard to TAble 53.2 anyone/all?

Whilst you are correct in what you say as Steps says with a TT system a 61009 device is not suitable for isolation referring to Reg. 537.2.2.1, referring to this and cross referring to nested regulations does still not require double pole circuit protection, or RCD.

537.1.4 A main linked switch or linked circuit-breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply on load & as a means of isolation.

A mainswitch intended for operation by ordinary persons, e.g. of a household or similar installation, shall interrupt both live conductors of a single-phase supply

Well here I take the BS EN60947-3 main switch meets the requirements for Isolation as it does indeed interrupt both Line & Neutral.

So far this is the only requirement for double pole switching, but only for isolation & not for automatic dissconnection under fault conditions. No mention of individual circuits needing to be isolated by means of DP devices as opposed to the whole installation.

I agree, the only requirement I can find is for double pole isolation, which the main switch meets.

537.2

Isolation is a function intended to make dead for reasons of safety all OR a discreat section of the electrical installation by seperating the electrical installation or section from every source of electrical energy.

The magic word here is OR, for a little word it can make a big change in your final installation choices.

Anyway as you see it doesnt say you have to isolate individual circuits, you can isolate the whole installation.

Again I would agree with this so far as I can see.

537.2.1.1

Every circuit shall be capable of being isolated from each of the live supply conductors. In a tn-s or tnc-s system, it is not nessesary to isolate or switch the neutral conductor where it is regarded as being reliably connected to earth by a suitably low impedance.

Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this.

This I feel is the one that people decide gives rise to the requirement of DP isolation of individual circuits...

But reading the first sentence it says "EVERY circuit" & not "every INDIVIDUAL circuit", so again I would say the main switch meets the criteria of this regulation.

Again M I agree with this and cannot find in any even nested cross reference any requirement for a double pole RCD, only double pole isolation, which we can achieve with the main switch, including on load emergency switching.

411.3.2 Automatic disconnection in case of a fault:

411.3.2.1Except as provided by regulations 411.3.2.5 and 411.3.2.6 a protective device shall automatically interrupt the supply to the line conductor of a circuit or equipment in the event of a fault of negligible impedance between the line conductor & an exposed conductive part or a protective conductor in the circuit or equipment within the dissconnection time required by regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4

Again no mention of supply types or the requirement to switch the neutral under fault conditions... only the line needs to be interrupted.

Remember you as the electrician need to look at the property type as well as think about the installation....for example a thatched building I would consider DP rcbo's due to rodents, electrical conductors, & the increased risk of roof fires

Again I agree wholly with this and cannot find anything to contradict.

Mind, for increased risk of fire double pole rcd would be required, however, there is NO reg that says this must be instantaneous!

Having said all that the above doesnt mean you disscount double pole RCBO's as a waste of time & money, they are a regulatory requirement in some installations...for example

Agricultural sites maybe an option to loose single circuits by means of DP rcbo as opposed to the whole installation when an RCD trips (705.422.7)

And the isolation requirements of 705.537.2 point to DP rcbo's as a way of complying.

I have not considered any Section 7 requirements, as IMHO we were not discussing these at this point, merely the requirements of a conventional domestic install.

Well I hope I've not made a numpty of myself :^O :Blushing

Seriously I do hope this has given a nudge to people to take a look at their BRB & make a decision for themselves..............Now ladies gentlemen I shall put on the tin hat & take cover. :Salute

My blue above as M107 had used the red.

So I can find no reason to discount his arguments and in fact they do seem perfectly valid.

I will ask my AE for his opinion as my annual visit is in July, on the 12th, so please remind me about this nearer the time!

Well, discuss...

 

[M107]

Thanks Sidey its nice to have somebody else give some input.

I'm ashamed to say the Corrigendum from 07/2008 I have found as a loose leaf in the draw :Blushing so apologies for the slight misinformation.

I have to say even me a regular (if not habitual poster) was nervous about posting on this subject, as it does seem to be one of the most asked/argued about subject across all sparking forums & untill I sat down and read the regs & cross ref'd the reg numbers (as sir sidey has) I have to admit I thought "may be all these people are right I've missed something & I'm wrong, DP rcbo's are required on TT system"

So if you see the argument on other forums & the posters are blanket advising (dare I say dictating...I've seen the posts so know what gets advised) that DP rcbo's are to be used.... please feel free to point them this way, where they will find an informed yes/no & why discussion with the end result being what I hope people will see as a difinitive "what the regs say!"

But as already said if you want to use DP rcbo's crack on there is nothing that says you cant, same goes for SP rcbo's

Thanks again Sidey.

Anyway .....Nurse the beer is gone I need anotherGuinness

 

[steptoe]

Steps,This only relates to my beloved BE2 mate sorry, NOT standard domestic TT!

how come sidewinder?

I dont see anywhere there is a caveat to that effect.

now, without that the only way I can see SP RCBOs being used is via the deviations box, which in itself is fine, use the main SW for isolation, but it is a deviation as it doesnt allow for inconvenience.

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that DP rcbo's are to be used.... please feel free to point them this way, where they will find an informed yes/no & why discussion with the end result being what I hope people will see as a difinitive "what the regs say!"

the regs are how you read them,

and I dont believe I am the only poster on here that thinks DP disconnection is required for TT if RCBOs are to be used as 30mA protection unless a deviation is stated,

or I may well be,

IMO I am in compliance, and until I have it proven to me otherwise SP disconnection with a main SW isolator is a deviation, only a code 4 mind, but still non-compliance.

 

[Sidewinder]

Steps,

Off out now I'll get the brb out again later and give you the xref regs that support my argument.

 

[Sidewinder]

OK Steps, have the BRB here now.

You referred to Reg. No. 532.1 (i) condition 2.

First section 532 is entitled Devices for protection against the risk of fire.

532.1 starts

Where, in accordance with the requirements of Regulation 422.3.9, it is necessary to limit the consequence of fault currents in a wiring system from the point of view of fire risk, the circuit shall be either:

(i) protected by an RCD complying with regulation 531.2 for fault protection, and

- the RCD shall be installed at the origin of the circuit to be protected, and

- the RCD shall switch all live conductors, and

- the residual operating current of the RCD shall not exceed 300mA

It goes on to discuss insulation monitoring devices, which are not really relevant.

Reg. 532.1 states where in accordance with the requirements of Regulation 422.3.9

 

[M107]

This is the best whys & wherefores of TT and rcd/rcbo's I've seen yet........ & not a sniff of bickering.

Well done all. Guinness

 

[Sidewinder]

All kinds of support & encouragement accepted!

 

[steptoe]

I can go with that sidewinder,

but how do we isolate for maintenance without causing inconvenience with the rest of the installation?

SP isolation is NOT permitted in TT, so to change a light switch we have to turn off the entire installation,

I cant see how this would comply with 314.1 parts (i) & (ii) .

I just dont see how you can use one main switch for isolation for general maintenance,

its hard enough sometimes to get people to allow you to turn 1 or 2 or 3 circuits off at a time, never mind everything.

 

[Sidewinder]

Ahh,

Now that is a different argument!!!

However without DP MCB's or RCBO's you would have to turn off a significant section of the install i.e. roughly half even with a so called 17th split board on any TT install.

That also may be inconvenient.

So you are correct with the minimising inconvenience argument, though that does not seem to hold with many scams.

They all seem to think that turning off a complete domestic install for maintenance is fine!

Realistically it is not that much of an issue is it?

Commercial or industrial TT installs may be a different matter, however, there is a get out.

This is in Reg. 411.5.2 Note 1 where by an RCD is not necessarily required in a TT install, where the Zs value is sufficiently low and reliable, this could be found in a steel structured commercial or industrial building.

 

[steptoe]

Ive done a few TT installs with 3 RCDs in a board.

yes, turning off half the house isnt ideal, but its a lot better than the whole house, especially if you are going to be a while, the householder will generally go to wherever they have power and remain contented,

plus the fridge/freezer stays on(even if it needs an extension).

on a whole house dead scenario they moan constantly about when its coming back on.

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Im going to ask about this tomorrow.

assessement time.

 

[Sidewinder]

OK cool, at least we are getting somewhere on this in general.

Please do ask and let us all know, mine is in 2m I'll ask too!

 

[a1]

What was the outcome Steps?

 

[Sidewinder]

This discussion started over PM the other night.

I have taken the relevant sections, there was some other chatter in the messages that is irrelevant.

A.N. Other:

I

 

[NozSpark]

I don't know where, but I'm sure that I read a few years ago that under fault conditions on a TT installation that all live conductors had to be disconnected,, it might have been BS7671, the onsite guide, the amicus guide or I may have just been dreaming

I do agree that you don't have to have RCD protection on a TT installation so long as you can guarantee a low enough Ra to ensure that your disconnection times a met

 

[steptoe]

Admin, NICEIC AC guy didnt come up.with a definitive answer, all down to individual circumstances,.

There was another issue brought up but I think Ill hold that until SW has some results from his discussion.

 

[Litttle Shonet]

Having read M107's post with regards to interpreting the regs, at first I disagreed with some of the points, then after reading them again and again, I started to agree with him.

Just goes to show that the BRB is open to interpretation.

But

Scenario: Customer(who knows a little bit about electrics) decides to change light switch himself, so switches off RCBO, thinking this will be adequate!!!

 

[wozz]

If the customer knows little about electrics why would they be changing a switch? Could they use a voltage indicator correctly? there may be more than one circuit in the switch, and the switch instructions which they should be reading may well say turn installation off at main switch.

 

[batty]

Having read M107's post with regards to interpreting the regs, at first I disagreed with some of the points, then after reading them again and again, I started to agree with him.Just goes to show that the BRB is open to interpretation.

But

Scenario: Customer(who knows a little bit about electrics) decides to change light switch himself, so switches off RCBO, thinking this will be adequate!!!

and why would this not be adequate?

 

[steptoe]

right, not much input into this,

so I will give one of the scenarios me and the assessor came up with as to why SP RCBOs would not be compliant under fault conditions,

daisy chain effect of SP RCBOs if a N-E fault had a low enough resistance.

any takers?

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and why would this not be adequate?

because its TT and the regs require TT to be isolated on all LIVE conductors.