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.

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.

AL said:

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.

Agreed ... just pointing out why, in many cases, it's not always an issue ... similarly, even if you can't achieve the 0.4 s or 5 s disconnection time, provided bonding resistances are low enough, it's not an issue ... meaning most control panel installations, it's just not a problem.

However, I can see there may be cases where you might want to make a distinction ... it's just not that easy to provide a simple "yes, always this" answer.

Have you had any further thoughts, after reading the replies so far, in relation to the cases you are looking at ?

Thank you both for your answers.

It seems that the correct answer is more nuanced than I originally thought. I know that standards can be a bit vague sometimes, which is why I was arguing that the circuit to be tested for Fault Loop Impedance should, in our case, be classified as a distribution circuit.

Seen as a whole, yes, the cabinet itself is a final circuit, a consumer, but I believe there's more to that. 

 Between the circuit that I initially queried about, External Supply - Main Circuit Breaker - AC/DC PSU, and a device that the operators can interact with, there are several other layers of protection that, in the event of a fault, must trigger first (downstream MCB, fused terminals, etc).

Furthermore, operators are always skilled personnel.

AL said:

Furthermore, operators are always skilled personnel.

Perhaps not always ... even here, there are shades of grey.

For example, on a water treatment facility, or for a machine that has a non-electrical operator, the technician that uses a panel-mount HMI (human-machine interface), might not have a complete suite of electrical competences, but if the panel just has indicator lights and controls that are intended for trained electrical or M&E personnel, then at least some manner of "skilled (electrical)" or "instructed (electrical)" might apply.

AL said:

Furthermore, operators are always skilled personnel.

Perhaps not always ... even here, there are shades of grey.

For example, on a water treatment facility, or for a machine that has a non-electrical operator, the technician that uses a panel-mount HMI (human-machine interface), might not have a complete suite of electrical competences, but if the panel just has indicator lights and controls that are intended for trained electrical or M&E personnel, then at least some manner of "skilled (electrical)" or "instructed (electrical)" might apply.

gkenyon said:

For example, on a water treatment facility, or for a machine that has a non-electrical operator, the technician that uses a panel-mount HMI (human-machine interface), might not have a complete suite of electrical competences, but if the panel just has indicator lights and controls that are intended for trained electrical or M&E personnel, then at least some manner of "skilled (electrical)" or "instructed (electrical)" might apply.

That is true, in this particular case, though, the panels are operated by trained personnel. The purpose of the system is for maintenance only.