Power or controls

JohnW2 said:

Put very simply, my interpretation/view is that any conductor(s) which carries the full load current (fully 'end-load' current) is/are part of the "power circuit" (not a "control circuit"), even if items within the current path can switch the current on/off (or even change its magnitude). On the other hand, anything (components and conductors) which 'controls' the load current (on/off or variable), without carrying the full load current, is part of a "control circuit".
Kind Regards, John

Click to expand...

I really wish it was that simple. I am happy to say that in essence I agree with this definition but unfortunately that distinction is frequently not there. A simple example would be to place a relay or contactor coil (Sorry for the autocorrect 'contractor' earlier) across the load in a power circuit (a not uncommon scenario) and that device then controls something else. Suddenly a power circuit by your description has been changed to a control circuit.

In real life the control panel and all the associated peripherals are classed as control.



Another example and this time a simple one, a heating company were called in to look at a heating problem and found a pump had failed.

They removed the pump and returned the following day with a replacement, fitted it and left it running.

That day the heating stopped again and a different engineer found the overload had tripped, reset it and turned the adjustment up to maximum.

Several days later the pump failed, it was replaced and returned to supplier under warranty.

Several days later the pump failed, and it tripped the overload. Overload reset and pump replaced but it kept tripping and diagnosed as faulty overload.

Electrician attended to replace overload (wrong make so it was a real botch to make it fit) it was again set to maximum (something twice the rated current) as it kept tripping.



Sunray was asked to have a look by a Controls company and was greeted with 'another person, do you reckon you can do any better than the other 6 visits?'

First job, take lots of pictures, lots of voltage and current measurements.



Any ideas?

It is an electrical issue, not plumbing.

Now we have the mess of a discussion spread over three threads

SUNRAY said:

I really wish it was that simple. I am happy to say that in essence I agree with this definition but unfortunately that distinction is frequently not there.

Click to expand...

As Simon has already highlighted, there a complication when a circuit is a 'dual purpose' one (power + 'control').. .

SUNRAY said:

A simple example would be to place a relay or contactor coil (Sorry for the autocorrect 'contractor' earlier) across the load in a power circuit (a not uncommon scenario) and that device then controls something else. Suddenly a power circuit by your description has been changed to a control circuit.

Click to expand...

Well, that's essentially the same as Simon's 'boiler and pump' example - a 'power circuit' has suddenly changed, not to a control circuit but to a 'dual-purpose' circuit (power+control) - and, as I said, your guess is as good as mine as to what that means in terms of the requirements of Table 51 of BS7671, since such a 'dual purpose' circuit is not a type of circuit for which they specify any (conductor identification) requirements!

SUNRAY said:

In real life the control panel and all the associated peripherals are classed as control.

Click to expand...

Yes, but in the real life of domestic installations (which is what that other thread started asking about) there are generally no 'control panels'. In any event, as I keep asking, what does "are classed as control" mean (electrically)? I understand your view that complex control seems may need to be dealt with by people with more specific knowledge than the average electrician - but, whatever else one calls that,. it's not a distinction based on any electrical considerations!

Kind Regards, John

SUNRAY said:

It is an electrical issue, not plumbing.

Click to expand...

Maybe that uninformed assumption is where the problem lies. A motor being supplied with volts and frequency within specification and operating one a reasonable duty cycle (not repeated stop-starts) is more than likely to be damaged by the mechanical load placed upon it. In the case of a pump, such loads are caused by the hydraulic circuit.

Jackrae said:

Maybe that uninformed assumption is where the problem lies. A motor being supplied with volts and frequency within specification can and operating one a reasonable duty cycle (not repeated stop-starts) is more than likely to be damaged by the mechanical load placed upon it. In the case of a pump, such loads are caused by the hydraulic circuit.

Click to expand...

There was nothing wrong with the mechanical side of things, I found the issue and corrected it and it was definitely an electrical error.

And the error was created by one or more of the previous repairs.

And it was ......

Well such a silly mistake, it was only a small pump, sizewise typical of a domestic central heating pump. IIRC about 150 to 200VA and the original wiring was 3 phase.
Sadly by the time I got there it had been changed 3 times and I found a single phase in place running on 400V.
An electrician had already mistakenly replaced the overload and I guess didn't do any investigation as to why the pump was running a high current. In the process he had also broken the contactors moulding to get the wrong overload in.
My repair was: change the contactor and overload and rewire for single phase, which meant it was running on red and blue wires with number idents.

There was another fault (on the alarm circuits) which had been overlooked which I fixed while there.

As I left the parting words were along the line of 'We expect to see someone else soon', but unlike the others I phoned them for several days to check.

SUNRAY said:

Sadly by the time I got there it had been changed 3 times and I found a single phase in place running on 400V.

Click to expand...

At what point during the saga do you think that that situation arose? If you're suggesting that it has 'always been like that', how long had it been 'working satisfactorily' before the six failures arose (in pretty quick succession)?

However, in view of the manner in which you introduced this story, I'm a little confused - what you describe was simply an 'electrical mess-up', rather than something which requires the specific skills of a control engineer, wasn't it?

Kind Regards, John

SUNRAY said:

Well such a silly mistake, it was only a small pump, sizewise typical of a domestic central heating pump. IIRC about 150 to 200VA and the original wiring was 3 phase.

Click to expand...

That whole explanation all sounded very confused indeed, especially so the bit about 400v and single phase.

I have seen some thing similar where an inverter was used, the inverter supplied three phase 230 volt to a motor wired delta, the inverter failed so as a temporary measure the motor was wired star and a 400 volt supply used, all worked A1. Then another inverter failed and the second electrician tried to copy what the first had done, but did not swap motor to star wiring.

These faults should never happen, 110 volt transformer replacement on a control circuit which is IT replaced with one centre tapped, electrician simply removed the neutral to earth link, without adding a fuse to what was now line two, resulted in full rewire of machine as all cables damaged by one neutral which burnt out.

I have worked with electricians who were well out of their depth, they had spent most of their live working on houses, and on the start of the job most of the work was similar to wiring a house, wire up a container as a workshop for example, then the batching plant, placing booms, concrete pumps, started to arrive, not the electricians fault, but he had never worked on three phase motors, had no idea how it all worked. There was enough single phase work that he did not need to work on the three phase, one of the other electricians would do that, and the foreman knew not to send the guy to do three phase work, however it was last in first out, so slowly all the other electricians left site, and he was only one left, I really felt sorry for the guy.

There are electricians who have never worked on three phase, and this has been made worse by part P law, as electricians would swap between industrial, commercial and domestic according to demand, but part P has generated a core of domestic only electricians, because scheme membership costs so much, electricians who do pay out, want to use what they are paying for so end up doing nearly all domestic.

We should learn in collage how three phase works, however when I went to collage I was taught how to use steam tables, but other than knowing they exist, today would not have a clue how to use them. As for a slide rule or log tables, likely I would remember once I started to use one again, but could not give some one else step by step instructions.

OK there are three phase machines in my house, but most domestic electricians will never touch them, only refrigeration guys would ever try to repair my fridge/freezer or freezer, and the multiphase fans in my PC are replaced as complete units. Think washing machine also has three phase motor, but again rather specialised and would involve complete unit replacement.

Harry Bloomfield said:

That whole explanation all sounded very confused indeed, especially so the bit about 400v and single phase.

Click to expand...

It did. It seemed to be saying that a 230V single-phase pump was connected between two phases of a 3-phase supply - but (per the questions I asked), I would not have thought that it would ever have lasted very long like that.

As I also said, I also couldn't understand what the story had to do with the point being made (about control circuits and control engineers).

I'm sure we'll get clarification.

Kind Regards, John

In the other thread, I mentioned that I'd be posting an additional wiring diagram, and here it is :


The change is the addition of a run-on timer for the pump.

If the boiler were replaced by one with a permanent live and a control (demand) input, then it would be clear that all of the timer & stat wiring would be a control system. In that case, we'd be free to use (eg) 0.75mm² conductors in the variety of colours in table 51.
But swap back to the boiler we have and ... nothing has really changed. All the wiring and devices are still protected by the same fuse, it's still all in the same installation methods (a mix of clipped direct, conduit, and trunking), everything still makes sense. Except that some of the wiring is now, by some definitions, "power" rather than "control" - meaning that it now needs to be massively oversized 1.5mm² (according to table 52.3) which makes no sense whatsoever in the context of a heating system and controls supplied via a 3A fuse

And if we look in section 2, the definition of "circuit" is (effectively) "everything on that diagram to the right of the isolator & fuse". So it's all one circuit, so it's all a control circuit, and it's all a power circuit

And for good measure, if the pump or boiler came with a fitted cable then it would most likely be 0.75mm². According to table 52.3, if the electrician supplies the cable then it has to be 1.5mm². Yep, it's looking more and more logical by the minute

As to "we are reading more into this that 'the book' says", well we have to read the book and try and abide by what it says. If it says "thou shalt do X" then it's not for us to start saying "well really they don't mean do X, it's obvious they meant to say Y". And if you are a member of one of the schemes (I'm not) then you have to abide completely with "the book" as a condition of your membership.
So it looks like a scheme member is required to use 1.5mm² for wiring heating circuits

Like I said earlier, I think the only thing we can completely agree on is that this is an almighty mess

SimonH2 said:

In the other thread, I mentioned that I'd be posting an additional wiring diagram, and here it is : ..... The change is the addition of a run-on timer for the pump. .... If the boiler were replaced by one with a permanent live and a control (demand) input, then it would be clear that all of the timer & stat wiring would be a control system. In that case, we'd be free to use (eg) 0.75mm² conductors in the variety of colours in table 51.

Click to expand...

… except that, as you go on to say, if the permanent L for the boiler were protected by the same OPD as the ‘control’ wiring, that would (per BS7671 definition) be part of the same “circuit” - hence, as I’ve said before, it would be a “dual purpose (power+control) circuit” (not just “a control circuit”), something which is not contemplated by either Table 51 or Table 52.3.

In the absence of either Table explicitly stating any requirements for a “dual purpose (power+control) circuit”, the most obvious approach would be to consider that, for such a dual circuit, the more onerous of the stated requirements for either type of circuit would prevail. Hence, if one required a higher minimum CSA than the other, the higher would prevail - and if one required (only) brown identification but the other allowed many other colours in addition to brown then the former (brown) would prevail.

SimonH2 said:

But swap back to the boiler we have and ... nothing has really changed. All the wiring and devices are still protected by the same fuse, it's still all in the same installation methods (a mix of clipped direct, conduit, and trunking), everything still makes sense. Except that some of the wiring is now, by some definitions, "power" rather than "control" - meaning that it now needs to be massively oversized 1.5mm² (according to table 52.3) which makes no sense whatsoever in the context of a heating system and controls supplied via a 3A fuse

Click to expand...

True, but what else can one/they do IF (without any apparently logical reason), they (seemingly arbitrarily) impose minimum CSAs for different 'types' of circuit, regardless of current and even OPD rating ?? As above, in such a situation, one imagines that the expectation is that a 'dual-purpose' circuit would have to satisfy the more onerous of the (arbitrary and seemingly illogical) required for the two 'types of circuits'. In fact, you don't need to have such a 'dual-purpose' circuit to encounter this (seemingly 'silly' issue....

... I have (for historical reasons) a circuit (originating from a B6 in a CU) which is essentially a dedicated supply to a hard-wired tiny load (a SMPSU about 20W). It is clearly not a lighting or control circuit so Table 52.3 requires a minimum cable CSA of 1.5mm², despite the fact that a very much small cable (maybe protected by a 1A fuse in an FCU, or even a B1 MCB {which I believe exist - B2s certainly do}) would, electrically, be perfectly adequate.

SimonH2 said:

And if we look in section 2, the definition of "circuit" is (effectively) "everything on that diagram to the right of the isolator & fuse". So it's all one circuit, so it's all a control circuit, and it's all a power circuit

Click to expand...

Exactly - as I've said it's neither a control circuit nor a power circuit but, rather, a dual-purpose (power + control) citrcuit - and I've commented about that above.

SimonH2 said:

And for good measure, if the pump or boiler came with a fitted cable then it would most likely be 0.75mm². According to table 52.3, if the electrician supplies the cable then it has to be 1.5mm².

Click to expand...

Not (if one ignores the nonsense created by their having forgotten to add “non-flexible” to the first section of Table 52.3) if it is (as it presumably it would be) flexible cable - since it can then be 0.75mm² (or ‘as specified by the product standard').

Kind Regards, John

SUNRAY said:

Well such a silly mistake, it was only a small pump, sizewise typical of a domestic central heating pump. IIRC about 150 to 200VA and the original wiring was 3 phase.

Click to expand...

JohnW2 said:

At what point during the saga do you think that that situation arose?

Click to expand...

As I read things, the timeline went along the lines of :

• System suffers a fairly basic failure (pump failed).
• Heating person correctly identifies this and removes the pump - but missed one crucial detail.
• Replacement pump is procured - but wrong one due to that missed detail - and fitted
• Everything goes to hell in a handcart due to that one detail and successive failures to identify the root cause.

And all that hassle, inconvenience, and expense because someone failed to notice that the original pump was 3 phase Hard to imagine how anyone competent to replace a pump could fail to notice the extra wire when disconnecting it And hard to imagine how anyone sourcing a replacement could fail to observe the 3 phase and 415V that would have been on the rating plate.
So one has to assume that the person taking the pump out was a complete numpty, the person sourcing the pump was also a numpty (probably didn't even look at the removed pump), and the person fitting the replacement was an even bigger numpty for not noticing the disparity in numbers of terminals. All three could be the same person of course.

ericmark said:

There are electricians who have never worked on three phase

Click to expand...

My guess would be that the numpty or numpties above are in that category.

JohnW2 said:

It seemed to be saying that a 230V single-phase pump was connected between two phases of a 3-phase supply - but (per the questions I asked), I would not have thought that it would ever have lasted very long like that.

Click to expand...

It does seem that a 240V single phase pump was connected across two phases and so over-volted by about 70%; and as described it didn't last long.

As I also said, I also couldn't understand what the story had to do with the point being made (about control circuits and control engineers).

Click to expand...

It's a bit tangential, but an illustration of how a little knowledge can be a dangerous thing. The heating "engineer" made basic mistakes; the following people (including someone supposedly an electrician) failed to do basic diagnostics; it took a control engineer with the right mindset to find the fault.

JohnW2 said:

Not (if one ignores the nonsense created by their having forgotten to add “non-flexible” to the first section of Table 52.3)

Click to expand...

Well as I pointed out, while we may assume that they "forgot to add it", we have to work with what they actually wrote, not what we think they probably meant to write. When what they actually wrote doesn't make sense, then that's a problem

if it is (as it presumably it would be) flexible cable - since it can then be 0.75mm² (or ‘as specified by the product standard')

Click to expand...

Ah, so the answer is to wire everything in flexible cables

But a serious point, if it's OK for the exposed flexible cable to be 0.75mm², it's hard to imagine why the same size of wire shouldn't logically be acceptable (subject to ratings, OPD rating etc) when nice and snug in a protective conduit or trunking.

SimonH2 said:

As I read things, the timeline went along the lines of : ...

Click to expand...

You may be right (and it's what seemed to most likely interpretation to me, too), but it can only be speculation ... which is why I asked SUNRAY for clarification (and await his reply).

SimonH2 said:

And all that hassle, inconvenience, and expense because someone failed to notice that the original pump was 3 phase ..... So one has to assume that the person taking the pump out was a complete numpty, the person sourcing the pump was also a numpty (probably didn't even look at the removed pump), and the person fitting the replacement was an even bigger numpty for not noticing the disparity in numbers of terminals. All three could be the same person of course.

Click to expand...

Quite so - and as I've already said, IF that was the situation, then I really don't understand SUNRAY using it as an example of why only "control engineers" should go near "control circuits" - despite the arguments about what constitutes a 'control circuit', replacing a pump correctly only requires someone with a modicum of electrical knowledge and common sense - and I really don't see why it requires a 'control engineer'!!

Kind Regards, John