separation of main power and safety circuit. I've been told off!

[Inteificio]

Hi,

I had an interesting conversation at work today which might cause a few problems.

I am fitting 4 pole MCBs to protect the 1.5mm 3 phase feed to some motors (There is no neutral).
The 4th pole is part of the control circuit.

My idea is that if the MCB trips, it will delatch the control circuit (and raise a fault alarm).

I have just been told that this is not allowed as their has to be electrical separation from control and power circuits.


In my eyes this is false as most latching circuits are fed from the contactor!

However in this regard the control circuit is 110v, so I thought I would check.

My defense is that even if separation is required there is total electrical separation between poles, only a mechanical link.

Does anyone know what the regs say on this, or can anyone offer some words of wisdom?

Thanks,

P

p.s. I should add although they -------ed me on this, they had no objection to using a D40 to protect the motor :thinking:

 

[telectrix]

i may be wrong here, but IMO, the separation is needed if the control circuit is elv. as 110V is LV, the i can't see a problem as both circuits are LV

 

[Darkwood]

Deleted......as i fear the OP is working in an area he isn't competent or experienced in.

 

[Inteificio]

front end device will afford short circuit protection both of which will cut power if circuit is designed correctly

The word 'design' and these circuits do not go together.

Correct me if I am wrong but an overload provides only overload protection and does not break fast enough to meet 0.4s for short circuits or earth faults?

The only protection is a D40, which will not trip fast enough/stop wires melting in either of these cases.

My solution was to provide a dedicated MCB for each circuit.

 

[shanky887614]

The word 'design' and these circuits do not go together.

Correct me if I am wrong but an overload provides only overload protection and does not break fast enough to meet 0.4s for short circuits or earth faults?

The only protection is a D40, which will not trip fast enough/stop wires melting in either of these cases.

My solution was to provide a dedicated MCB for each circuit.

1.5 on a d40?

I think you got another issue there mate

 

[netblindpaul]

If you are into motor control, then forget about BS7671.
No issues with D type breakers.
Not quite sure what you are trying to do, but you need to refer to BS EN 60204-1, and the statute law regulations along with the associated type 1, 2 & 3 standards for the machinery that you are dealing with to ensure it complies with statute law.

 

[Marvo]

Once you're insidea control panel the installation regulations don't necessarily apply so the curve and ampacity of the MCB could be part of the controls design.

*Edit* Beaten to the post by Paul

 

[Darkwood]

Deleted....

 

[Inteificio]

I always forget rules of this forum. Whenever anything less than 100% of the info is given, threads will always shoot of at a tangent!

So as this is going to go a bit more in depth, I should add this is a VERY high risk room. Damage to the electrics happens frequently and contact with live wires is quite possible at all points on every circuit.

I'm trying to make this room safe. So wanting to clarify a few points. There are MANY problems with this install, the D40 is just one of them.


Some interesting points in the text above!

"1.5 on a d40? - I think you got another issue there mate" Hell yeah. I completed over 2 pages of faults for this one room. Sy sheath is normal CPC for this room too, this is when they even bothered to try fitting one.



"If you are into motor control, then forget about BS7671." - This is an interesting point. I am doing the repair to make it safe. However where do the requirement stop? Some people say as far as the motor, others the feed to the board. Anyone know for certain?

In this regard as people are climbing on the motors, on the isolators and walking on the wires the entire install should be safe and meet the safety requirements of regs.

"No issues with D type breakers" - It does depend on the circumstances. If the Zs is out and the calcs say the wires are going to melt, then I do have an issue. There is nothing wrong with a correctly specced D though, I will always try to put in a C or B if I can due to the increased safety ( a short is not always a low resistance short).


"I think you are confused here" - Quite likely, please bear with me I want to make sure I'm doing the right thing.

Zs is WAY out, so D40 will not trip in time (without cooking the wires)
Am not worried about overload, the overloads cover that.
"fusing down any wire when necessary" - That's what my C16s are doing.
"cable is most likely undersized for installation calcs" - I can guarantee that there were no calcs done when this was installed. I would be surprised if the installer had any electrical training...or could even count without using his 11 fingers.


Am not sure if I mentioned above, I didn't do the original install. Some cowboy contractors did the work and has been endangering the lives of every user of that room since then. When I joined this company I spotted the problems. Did some dodgy repairs with zero budget and have been arguing to fix it since then.
Company has finally agreed for me to make the room safe. So that's what I'm doing.

 

[netblindpaul]

So we get half a story, oh & SY braid is not acceptable as a CPC.


Yes the whole install must meet regs, it is just WHICH regs it must meet BS7671 is not the be all and end all.

TBH, I can't really make head nor tail of your issues, so it is probably best I say no more.

 

[Inteificio]

"SY braid is not acceptable as a CPC." - You don't say ;-)

Net your help was appreciated above, I will ask work about getting BS EN 60204-1.

 

[shanky887614]

So we get half a story, oh & SY braid is not acceptable as a CPC.


Yes the whole install must meet regs, it is just WHICH regs it must meet BS7671 is not the be all and end all.

TBH, I can't really make head nor tail of your issues, so it is probably best I say no more.

Hmm zs is too high, surely it would be better to tackle this first, bigger cable for supply cpc me thinks

 

[Tiny Spark]

My first question is why are people climbing over the motors and isolators?

You say this room is "high risk" why is it high risk? Is it high because stupid people do stupid things like climbing on isolators or its high risk because of the environment or just the plain installation?

I get the feeling there is somthing bigger here than just the electrics, If the place is that dangerous something should be done about it anything that is risky should be reduced as much as possible using common sense and risk assessments.
If your employer does not do that the HSE will have a field day with them if anything goes wrong.

As for your problems I think you need to give us a little more info on the actual control and wiring of these circuits as tbh ime a little confused.
not that I pretend to be any good when it comes to panel wiring but I know a few on here are!

 

[bugsy]

1.5 on a d40?

I think you got another issue there mate

I was beginning to wonder1

 

[Inteificio]

"You say this room is "high risk" why is it high risk? Is it high because stupid people do stupid things like climbing on isolators or its high risk because of the environment or just the plain installation?"

All of the above.

The main issue is using isolators as steps to reach high up things. You would be amazed how often this happens.



"a little more info on the actual control and wiring of these circuits as tbh ime a little confused."

SWA feed to panel -> main D40 breaker -> C16 breakers for each circuit (new) -> Contactor -> overload -> SY between panel and local isolator -> Isolator -> SY-> motor.

Motors a spread all over room.

The control circuit latches the contactor with a 110v. The latch originally went through e-stops, safety switches, and overloads. I have modded the circuit so this latch also goes through the isolator and MCB.

So if any part of the circuit has a fault the whole thing delatches and powers down. One recent electric shock was from an electrical engineer who was repairing a safety switch when it was isolated. He got a nice 70v hit from an 'isolated circuit'.

I could spend all day explaining all the issues. Really mainly interested in my original question.

Is there any reg which prevents multiple circuits being passed through one multipole MCB?