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

[Mark42]

I'm refurbishing my own workshops and fitting a new central three phase compressor and three phase dust extractor.

Both these appliances will be controlled by remote START/STOP switches, next to multiple compressed air and vacuum points, which may be in different buildings.

For future-proofing I want to have more than one control station for each appliance, for convenience when moving machines around the workshops.

Using what I believe is standard wiring for a three pole + aux n/o contactor (sketch below) it’s easy enough to use the existing three core and earth cabling for one station only, latching the contactor.

But to allow additional remote stations I can’t think of a way to do it without using four core + earth. An unlimited number of n/o START switches can be installed in parallel, but the STOP switches need to be in series as they are n/c.

Of course simple on/off switches powering the contactor would work, but that would be a terrible idea as you’d have always to switch off from the same position as you switched on.

Is there some clever way of making the wiring easier and clearer than having a long loop for the stop circuit? This would be particularly confusing if the geometry of the cabling is not simply point-to-point but partly a 'star' configuration.

I’ve not done this before and have no idea if there’s some standard method I’ve missed.



Thanks, Mark

 

[UNG]

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

 

[Mark42]

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

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!

 

[UNG]

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

 

[Darkwood]

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.

 

[Mark42]

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.

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!

 

[Darkwood]

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.

 

[brianmoooore]

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.

 

[Darkwood]

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?

 

[Darkwood]

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.