Multiple control stations for one three phase compressor. Best practice?

If you daisy chain the remote stop / start buttons then it could be wired in 3 core and earth

UNG said:

If you daisy chain the remote stop / start buttons then it could be wired in 3 core and earth

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Yup got it - one n/o circuit, one n/c circuit, common return. I agree that works fine with a straight point-to-point daisy chain.
I was getting confused with trying to work out something which allowed a star configuration as well. I guess it's impossible without 4+ cores. Unless anyone can think of something!

I drew exactly the same drawing before I posted my first reply
I think a star configuration could get messy and create unnecessary problems especially on the stop circuit

If you have more than 2 actuators (limits, stops etc) the you will fall foul of other regulations and will need to have some kind isolating TX or a dc power supply. It isn't just about designing a functional circuit but also complying to machine control regulations given you controlling a motor here. Just a heads up that there is more to designing this than I believe you are aware of, having said that, if this is for your sole private use at say your home then as long as its safe then you don't have to adhere to such regulations.

Darkwood said:

If you have more than 2 actuators (limits, stops etc) the you will fall foul of other regulations and will need to have some kind isolating TX or a dc power supply. It isn't just about designing a functional circuit but also complying to machine control regulations given you controlling a motor here. Just a heads up that there is more to designing this than I believe you are aware of, having said that, if this is for your sole private use at say your home then as long as its safe then you don't have to adhere to such regulations.

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Useful info, thanks.
The machines are:
1. A dust extractor for woodworking machinery, which is just one motor driving a centrifugal fan. Effectively a big vacuum cleaner plumbed in to fixed ducting with blast gates at each connection point. The machine has no interlocks or actuators at all.
2. A standard commercial screw air compressor, entirely contained within a metal box with no access to moving parts.
All my switches will do is to move the control points (including, importantly, emergency stops) from the face of the remote machines to more convenient positions at individual workstations. If that's at variance with some obscure regulation, then the regulation is daft!

The E-Stops are classed as actuators assuming they are actually run by an E-Stop safety control circuit otherwise they are simply functional stops regardless of them looking and operating like an E-Stop. Either way they are still classed as actuators, if they are been used in anyway for safety then a risk assessment and a E-Stop system meeting the required category would be needed.
You simply can't just add more starts and stops, to do this kind of work you should be fully converse with the regulations that govern them as this falls far outside the scope of the BS7671, it is one of the most common issues I come across seeing non compliant work done, if anyone else other than you is operating this equipment or this is for a customer then I will also advise you to inform your insurance company of this work as it is unlikely you would be covered unless you specified you do machine safety and control... you will know if your covered because your insurance will be substantially higher than the going rate for insurance of an electrical contractor.
PS - your statement of the regulations may be daft only shows your ignorance here, these are critical safety circuits and one should leave well alone unless as mentioned you have knowledge and understanding of the overlying regulations.... that comment alone would be on par with a customer saying to you 'what is the point of having different rated fuses, simpler if they were all the same'.. hope you appreciate there are reasons for such regulations that you are probably unaware of.

Since these machines are fully enclosed and have no accessible or potentially accessible moving parts, surely these are just on and off controls, not emergency stops, so don't come under these particular regs.

As I said originally, it is dependent on the situation and whether these are classed as functional remote stops or for any reason that has not been disclosed there exists some form of risk that would warrant an upgraded system otherwise an extension of the start/stop function is fine but this still needs to be done to relevant regulations which may require additional control equipment.
A response from the OP clearly stated emergency stops, this is why I followed with the previous response, does he perceive a risk that warrant emergency stops or is he intending on installing latching stops and is simply mislabelling them as E-Stops because they are normally used as such?

I just got back home and delved into the BS 60204-1 for which the OP should comply to in doing this work.

9.1.1 Control circuit supply ( cropped version for the above discussion)
- Where control circuits are supplied from an A.C. source ,control transformers shall be used for supplying the control circuits.........

-Transformers are not mandatory for machines with a single motor starter and a maximum of two control devices (example - interlock device, start stop station )

This is the regulation you will be breaching and the reason why at the moment on its own the compressor and extract system do not require a TX in the control and is as stated because they do not exceed the max control points (2) in their design, your intentions will pass this limit thus you will have to upgrade the control circuits for both to meet requirements, if this is for the OP as he implies and only used by himself say in his own private workshop then there is no real requirement to adhere to these regulations (although I would advise against that), however if others are to use these machines and operate these controls then not only will they have to be fully compliant but he will be taking on board all responsibility for the machinery and its control system as you have voided the manufacturers warranty by adapting their system, as mentioned also this would also require him to ensure his insurance covers him for this work which will be unlikely if he has standard electrical insurance policy like PL.

I agree this may seem overkill if one is not knowledgeable about machinery control systems and safety implications etc but there are good reasons for these regulations especially in environments like workshops which increase the risk to operators thus the need for some form of isolating TX.
Don't shoot the messenger here, I am giving constructive advise based on the regulations, you may not like it but it is what it is.

Any particular requirements for the isolating Tx, other than it is an isolating type, with the windings on separate legs or an earthed screen between them?
Does it need to reduce the voltage or is 230:230 OK?
Shouldn't be to expensive to make this compliant. Tx will only need to be 10s of watts.

Emergency stops, if originally fitted to the equipment, will need to be re-implemented, otherwise the person modifying the control system, will become the manufacturer of the equipment in law.
Any control circuit modifications will need to comply with the relevant requirements of the law, PUWER & SMSR.
They require compliance with the relevant designated standards.
As has already been mentioned BS EN 60204-1, additionally BS EN ISO 13850 for emergency stop functions.
BS EN ISO 13849-1, with validation by an independent person (not the circuit designer) in accordance with BS EN ISO 13849-2.
These are far from obscure standards, they are the underpinnings of machinery safety legislation.
You can forget BS 7671, clause 110.2, xi, excludes machinery controls.
The Designated Standards have a much closer and stronger link to the relevant legislation than for example the statement by HSE in the front of BS7671 because both the standards and legislation are designed and enacted to support and reference each other.
The other thing is the insurance aspect, both for the work on the controls and the insurance for the location where the machinery is being used, business or home.
Finally, the law is constructed or modified such that machinery for the use within a business undertaking even when the machinery is designed and manufactured or modified by the user falls within the requirements of the law.

netblindpaul said:

Emergency stops, if originally fitted to the equipment, will need to be re-implemented, otherwise the person modifying the control system, will become the manufacturer of the equipment in law.
Any control circuit modifications will need to comply with the relevant requirements of the law, PUWER & SMSR.
They require compliance with the relevant designated standards.
As has already been mentioned BS EN 60204-1, additionally BS EN ISO 13850 for emergency stop functions.
BS EN ISO 13849-1, with validation by an independent person (not the circuit designer) in accordance with BS EN ISO 13849-2.
These are far from obscure standards, they are the underpinnings of machinery safety legislation.
You can forget BS 7671, clause 110.2, xi, excludes machinery controls.
The Designated Standards have a much closer and stronger link to the relevant legislation than for example the statement by HSE in the front of BS7671 because both the standards and legislation are designed and enacted to support and reference each other.
The other thing is the insurance aspect, both for the work on the controls and the insurance for the location where the machinery is being used, business or home.
Finally, the law is constructed or modified such that machinery for the use within a business undertaking even when the machinery is designed and manufactured or modified by the user falls within the requirements of the law.

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Fancy seeing you here Mr S ? it's bad when you read a post and think, I know exactly who would write like that ?

This is getting silly.

If all I'm supposed to do is to use an isolating transformer, or ELV circuit, for the remote controls (when there are more than two switch positions) that’s trivial to implement.

But why do it? If two directly-connected mains voltage control stations are safe, why aren’t three? Or a hundred and three? Genuine question.

Maybe I’ve introduced complexity by using the term ‘emergency stop’, which I accept may have an unintended meaning in law. What I really mean is a simple off switch. Ignoring semantic argument, in general language the latter can act as the former.

The most-used machine is a simple chip extractor. (Pic below). This is not some industrial production line with entrapment dangers, exposed mechanisms and rotating parts. It’s a bloody vacuum cleaner: sold widely for commercial and hobby use.

I'm struggling to understand what hazards such a machine could present.

If the regulations prevent me from adding remote switches next to the saws, drills etc where the vacuum ducting ends, it would be necessary to walk into the next room - and sometime the next building - every time an operator wants to cut even one small piece of wood, then return again to switch the extractor off. This obviously means sometimes an operator wouldn’t bother, creating a dust hazard. Any plan which does not include human factors is an incomplete plan.

Also I’d have though it better planning to have a way of turning the vacuum off immediately if something big gets sucked up and jams the pipes or fan. Not particularly dangerous, but it could damage the machine.

These machines are sometimes provided with cheap remote controls of the type used for garage doors, sold without limit to number, for hanging on rusty nails driven into the wall next to each machine. How is that different (or better/more compliant) than what I propose?

Let's first get things right here, regulation is not preventing you doing what you want to do in this case, it is merely stating the preferred method you originally wanted to take would not comply and you need to implement certain design features to do the work and comply so you can achieve what you wanted to do simply not the way you intended.
Yes on the preface of it it does sound a crazy regulation but you have to look at this the opposite way around in that the regulation is applicably the normal but your approaching this from an exemption to this regulation.
I would also like you to give citation of any 'machinery' like the one you exampled above that has this remote accessory you make claim to, I find no option on Charnwood site for this.

To give you incite to the regulation and why they are applied you need to understand the environments this equipment is designed to be used in, these are he likes of workshops, factories etc, these are areas where risk of damage, contaminants, accidents are perceivable high and thus regulations are brought in to try keep operators safer, consider a 3KW motor connected directly to a start/stop DOL contactor in a contained enclosure similar to the one mounted on the picture you posted -
The control has no fusing down protection
The control has no isolating TX
The control is low voltage and not SELF
The exemption that is put on this set-up is basically because the risk assessment of the fully all in one enclosed system does not raise the risk to operators even in fault conditions, however the moment you start to adding external control points, limits, start/stop station you are creating added potential for damage, abuse, misuse etc and this exponentially increases the risk to users so that is where certain regulations kick in, the one I mentioned is one of many you would need to comply with here.
Let's look at this another way, if you had a workbench and a grinder on it you could have it on a flex and 13amp plug plugged into an adjacent socket the risks of damage although not omitted are small, if however the socket was three benches across and you wanted to add a local socket for convenience then that socket, it cabling etc would need to comply for installation practices for its environment and the risks it poses, IE you couldn't simply run a flex across the 3 benches and fix a socket locally, it is likely if you adhere to even the BS7671 on this matter you would house the extended circuit in impact resistant enclosure like conduit or trunking.
Regulations are based around real world risks, real accidents and historical data gathered over the period of the existence of regulations, you may have the most obedient and respected employees who look after the equipment and are class A safe and thus never damage anything, never operate a machine with wet hands and never trip up and grab what might be a wire connected to a control system and yes your install may never see issue in its entire life of operation but that unfortunately isn't how regulations work, they are designed on risk assessment and averages.

We are merely the messengers here so try not to get frustrated with us, I build machine controls for a living, I repair them and upgrade them and I fully understand why these regulations exist and I could show you many examples of damaged control stations that could have been a serious safety hazard had the regulations not been followed so I struggle to understand your position when you call it silly, although I empathise slightly because when I started in this side of the trade it too was a learning curve and wasn't until I saw many occasions the regulations kept operators safe that I lent them more respect and understanding.