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.

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.

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.

The section I would look at is Annex A of 60204-1,

"A.1 Fault protection for machines supplied from TN-systems

A.1.1 General
The provisions in the Annex A are derived from IEC 60364-4-41:2005, and IEC 60364-6:2006.
Fault protection shall be provided by an overcurrent protective device that automatically
disconnects the supply to the circuit or equipment in the event of a fault between a live part
and an exposed conductive part or a protective conductor in the circuit or equipment, within a
sufficiently short disconnecting time. A disconnecting time not exceeding 5 s is considered
sufficiently short for machines that are neither hand-held nor portable.
Where this disconnecting time cannot be assured, supplementary protective bonding shall be
provided in accordance with A.1.3 that can prevent a prospective touch voltage from
exceeding 50 V AC or 120 V ripple-free DC between simultaneously accessible conductive
parts.

NOTE The use of supplementary protective bonding does not preclude the need to disconnect the supply for other
reasons, for example protection against fire, thermal stresses in equipment, etc.

For circuits which supply, through socket-outlets or directly without socket-outlets, Class 1
hand-held equipment or portable equipment (for example socket-outlets on a machine for
accessory equipment, see 15.1) Table A.1 specifies the maximum disconnecting times that
are considered sufficiently short."

I added highlights to the excerpt. So my reading is for a TN supplied system, 5 seconds for fixed panel, or 0.4 seconds for portable as Table A.1 mentioned is the same as Table 41.1 in 7671. But you may need to read more as I dont know your full circumstances on the system.

The section I would look at is Annex A of 60204-1,

"A.1 Fault protection for machines supplied from TN-systems

A.1.1 General
The provisions in the Annex A are derived from IEC 60364-4-41:2005, and IEC 60364-6:2006.
Fault protection shall be provided by an overcurrent protective device that automatically
disconnects the supply to the circuit or equipment in the event of a fault between a live part
and an exposed conductive part or a protective conductor in the circuit or equipment, within a
sufficiently short disconnecting time. A disconnecting time not exceeding 5 s is considered
sufficiently short for machines that are neither hand-held nor portable.
Where this disconnecting time cannot be assured, supplementary protective bonding shall be
provided in accordance with A.1.3 that can prevent a prospective touch voltage from
exceeding 50 V AC or 120 V ripple-free DC between simultaneously accessible conductive
parts.

NOTE The use of supplementary protective bonding does not preclude the need to disconnect the supply for other
reasons, for example protection against fire, thermal stresses in equipment, etc.

For circuits which supply, through socket-outlets or directly without socket-outlets, Class 1
hand-held equipment or portable equipment (for example socket-outlets on a machine for
accessory equipment, see 15.1) Table A.1 specifies the maximum disconnecting times that
are considered sufficiently short."

I added highlights to the excerpt. So my reading is for a TN supplied system, 5 seconds for fixed panel, or 0.4 seconds for portable as Table A.1 mentioned is the same as Table 41.1 in 7671. But you may need to read more as I dont know your full circumstances on the system.