Final circuit definition BS 7671 / IEC 60364

AL said:

Can the circuit between a distribution cabinet (say 1FC1 MCB) and a control cabinet (1FC1 MCB - 1TA1 AC/DC PSU) be considered a final circuit, or is it just a distribution circuit?

That depends on the circumstances.

AL said:

I'm looking for an outside opinion on the "Final circuit" definition.

No-one can provide a definitive interpretation, except a court of law.

To give an opinion, some more details are necessary as to the arrangement and circumstances.

For example, what does the control panel do? The circuits from the control panel, in some instances, could be considered 'auxiliary circuits' and these wouldn't strictly be 'final circuits'; in other circumstances today, the control panel is a data communication device only, so is likely to be potentially considered current-using equipment.

On the other hand, is it part of 'machinery' as defined in the Supply of Machinery (Safety) Regulations?

The expert providing the opinion might also wish to know what standard(s) the control panel is manufactured or assembled to.

AL said:

Can the circuit between a distribution cabinet (say 1FC1 MCB) and a control cabinet (1FC1 MCB - 1TA1 AC/DC PSU) be considered a final circuit, or is it just a distribution circuit?

That depends on the circumstances.

AL said:

I'm looking for an outside opinion on the "Final circuit" definition.

No-one can provide a definitive interpretation, except a court of law.

To give an opinion, some more details are necessary as to the arrangement and circumstances.

For example, what does the control panel do? The circuits from the control panel, in some instances, could be considered 'auxiliary circuits' and these wouldn't strictly be 'final circuits'; in other circumstances today, the control panel is a data communication device only, so is likely to be potentially considered current-using equipment.

On the other hand, is it part of 'machinery' as defined in the Supply of Machinery (Safety) Regulations?

The expert providing the opinion might also wish to know what standard(s) the control panel is manufactured or assembled to.

Thank you for the reply.

The control panel contains an AC/DC PSU and a PLC rack.

To this PLC rack, we have several field devices connected, such as Safety Interlock Gates, Pushbuttons, Signaling Beacons and Sensors, all 24 VDC supplied. 

The AC supply branch doesn't extend beyond that one PSU.

You could look at this both ways.

The PSU converts electrical energy to electrical energy ... so is not, by definition 'current-using equipment'.

Some of the 'field devices' do, however, do so ... so the control panel definitely supplies final circuits.

You did not provide which standards the control panel is manufactured or assembled to, but it could be classed as a 'switchgear or controlgear assembly' (BS EN 61439 series, or its predecessor BS EN 60439 series), or 'electrical equipment of machines' (BS EN 60204-1)

One could take the view, if the former standards (EN 61439 / EN 60439), the AC supply to it is more likely to be considered a distribution circuit, whereas the latter standard, the 'machinery' as a whole could well be 'current-using equipment', and therefore this, being part of 'machinery' could make the AC supply to it a final circuit.

On the other hand, you could decide the PLC converts some electrical energy into heat ... so it's current-using under all circumstances. However, pedantically, is the circuit that supplies that inside the panel protected by an Overcurrent Protective Device in the Panel? If so, that is the final circuit, and the supply to the panel is a distribution circuit.

The question you might prefer to ask, is not 'does this meet the legal defintion of ?' nor the related

'if I had to stand up in court could I defend treating it this type of circuit ?'

But rather, as it answers at least the second one, consider  what is any peculiar odd-ball case most like ?
Does it pose the same risks as a final circuit  - are folk coming into contact with appliances, or operating controls or switches and fittings that are connected to it ? - If there is a credible fault, are the risks of a shock more or less than those of  a distribution cable?
Or not. 
Submains can be given 5 second protection, because there is something faster nearer the load end of the line protecting the user. 'Coarse' and 'fine' protection are no longer phrases commonly used but the old idea is perhaps still one to keep in mind.

Treat the installation as the situation it most represents.  If the ADS covering this wiring is the last line of defence before something that will be handled/operated by a normal user, you should very probably afford  it the same protection as a final circuit. 
Mike. 

Similarly, you could take the view, as to, similar to a switchboard, where, if such a control panel is usually accessed by skilled and/or instructed persons, a similar disconnection time (5 s or 1 s) might be anticipated ?

gkenyon said:

On the other hand, you could decide the PLC converts some electrical energy into heat ... so it's current-using under all circumstances.

Just like any cable or switchgear, etc.

Is a relay current-using equipment?

I think that the purpose of the equipment is important. "electrical equipment intended to convert electric energy into another form of energy" (my emphasis).

I am less clear on control gear which has some form of display, given that the light which is emitted is intended rather than being merely incidental.

It is the latter of the 2 standards BS EN 60204-1.

Apologies, must've missed clicking reply.

AL said:

It is the latter of the 2 standards BS EN 60204-1

Does it have a separate bonding connection to local protective earth bar, or MET ?

(As usual where control circuits are used with screened cable?)

If so, perhaps there's less of a problem with voltage differences ... basically, bonding and resistance of cpc's and bonding to adjacent equipment, may well meet the requirements for Supplementary equipotential bonding, which is used where disconnection times can't be met? over-rides the need to consider a difference between 5 s and 0.4 s disconnection times, as the resistance values are based on a current causing a disconnection time of 5 s only?

However ... regardless of bonding ... if the AC circuit is supplied by a Type B or Type C circuit-breaker, the 0.4 disconnection time EFLI is the same as a 5 s disconnection time EFLI ? It's this last reason why we really often never bothered to get to the bottom of this question in respect to most of the control panels that I was involved with ... with the use of such circuit-breakers, the question becomes a moot point.

gkenyon said:

Does it have a separate bonding connection to local protective earth bar, or MET ?

MET

gkenyon said:

However ... regardless of bonding ... if the AC circuit is supplied by a Type B or Type C circuit-breaker, the 0.4 disconnection time EFLI is the same as a 5 s disconnection time EFLI ? It's this last reason why we really often never bothered to get to the bottom of this question in respect to most of the control panels that I was involved with ... with the use of such circuit-breakers, the question becomes a moot point.

It's possible that the MCB upstream is a Type D, where a 0.4s disconnection time kicks in, and the differentiation between final and distribution circuit is important.

 similar to a  switchboard, where, if such a control panel is usually accessed by skilled and/or instructed persons, a similar disconnection time (5 s or 1 s) ..

Exactly so ! - it all rather depends on how it is used and by whom.; as that is actually what defines the risks, that drive the decision about what ADS time is appropriate.
Hence my comment about seeing it as part of a bigger picture, rather than black and white.
Mike.