No Volts Protection

sparkyparky · Apr 22, 2014

Hi guys I've got a customer that's changing a centrifugal exhaust fan from a single phase version to a 2.2kw three phase version. This is currently run through a rotary isolator into a contactor with the control run through a timer no DOL present.
My question is does something like an exhaust fan require no volts protection? If it does then I'm assuming you wouldn't be able to have a timer involved unless the DOL starter fed a Contactor which was then controlled by a timer.
How do you know what types of machine do need it (unless it is all types) excepting the obvious ones like those with exposed moving parts etc. Any help or clarification would be greatly appreciated!

cheers!

netblindpaul · Apr 22, 2014

Yes it does, plus it needs a control system to comply with the relevant requirements of statute law.

davesparks · Apr 22, 2014

If I remember correctly then when I went to college a starter was required for any motor over 1HP (~0.75KW).
But our lords and masters have changed the colour of the book a couple of times since then so this may be different now.

And as the gentleman above has mentioned there are other rules to be obeyed here, which are actually laws unlike BS7671.

Darkwood · Apr 22, 2014

Reg 552.1.2

Every motor having a rating exceeding 0.37KW shall incorporate a control system incorporating overload protection.... this applies to the OP's set-up although their are exclusions to this reg but would only bring confusion to list them ...

Reg 552.1.3 Also addresses protection against motor restarting after supply failure.

You should be able to fit an all in on start /stop unit with interlock switch, if the controls require more options or remote operation points etc then you need to have a good grasp of the BS60204.

Im going to hazard a guess that by the nature of the question this is not your field of experience so tread carefully this may be a simple resolve on this occasion but it may insight a confidence that will get you in too deep before you realise it if you continue to accept work of this nature... If it interest you I would work along side a competent sparks/engineer for a good few years before stepping over this line.

Knobhead · Apr 23, 2014

Most modern thermal overload units incorporate single phase protection.
A bog standard off the shelf DOL starter will have the O/L’s fitted.
There is no real need for under-voltage protection because a mains failure or a noticeable voltage will drop the contactor out.

davesparks · Apr 23, 2014

The DOL provides the under voltage protection as it won't automatically restart when voltage is restored, it is one of its functions. Whereas a contractor and timer does not as it could automatically restart when the voltage is restored.

Yes I know you know that already Tony, but it may not be clear to the op.

sparkyparky · Apr 23, 2014

Ok guys that's great thank you all for your help, it turns out he doesn't use the timer anyway so it'll just be just a standard DOL which keeps things simple not that it sounds like he would have been keeping it anyway.

It's not my area of expertise but the place where I did my apprenticeship/worked until recently was commercial/industrial biased so I'd like to think I know enough to know what I don't know, if that makes sense. I just never had the opportunity to get massively involved in this side of things (machines and motors etc.) and wouldn't do anything more complicated control wise than this it's just the timer set alarm bells ringing.

Just a couple more questions if you don't mind, out of interest which are the statutory requirements that you're referring too? And again out of interest how would you have this sort of thing timer controlled, could you have it so that they're just switched off by the timer say if they needed them to run for a bit after they've gone home?

Thanks again!

Darkwood · Apr 23, 2014

Your intergration of a timer changes the whole risk assessment of the set-up as you now have a motor/fan that can fire up without notice if so timed and/or over-run.

Extra safety measures maybe needed to ensure maintenance and servicing can be done safely, the timer must not act like a primary control and must come secondary to any stop commands or failures. Example the timer outputs will be calling for fan to run even after power failure and this should not start the fan up again when power is restored unless full risk assessment has been done and measures implemented to ensure its safe to do so in all circumstances..... without seeing the set-up its random shots in the dark Im giving but you need to tread carefully when adding anything to a basic all in one control.

Its very simple to adapt a timer to give over-run and/or time start but strict regulations are required to be met for safety reasons - the last thing you would want is someone clearing out the fan duct and the timer pulls in either because clocks have changed or the operative forgot.

sparkyparky · Apr 23, 2014

Oh right I see that's great again thank you very much for your time. One final question if you don't mind and I may very well be wrong but hypothetically speaking could you have a primary contactor/dol feeding a secondary contactor which is then timer controlled so if there is a power cut etc the primary will have to manually closed before anything else can happen? I'm assuming the primary contactor would also need to switch the outgoing neutral.

Thanks again!

Darkwood · Apr 23, 2014

Normally your E-Stop Safety circuit is your Primary control circuit which will usually control a main contactor via a manual activated reset, the secondary circuit then is the Start/Stop control circuit... as you don't have an E-stop set up or safety circuit as such (It may not be required for a simple set up like controlling a extract fan - subject to scenario) I assigned the primary to your main start/stop option this without the timer will all function correctly - when you introduce a timer you need to ensure its suitable for the job - or design the controls to suit... I gave a standard timer as a secondary control because its automatic switching it should not take priority over the start/stop buttons and shouldn't re-energise the contactor after a power failure unless steps have been introduced to ensure this is safe to do at all times.

A standard timer with have N/O and N/C switching contacts that when timer is calling they stay put in their respective position... the start option on the contactor is only momentary though by pressing the start to latch in the contactor via the latching contact..... if you were to wire the contacts in the timer to pull in the contactor it will remain energised through a closed timer relay and not the latching contact. This would have a consequence that in a power failure the contactor drops out but as the timer has battery backup or mechanical memory it will re-energise the contactor immediately on power supply been returned.

Also to watch out for is how you intergrate the timer - if the circuit is wired so timer contacts is parallel to the start button then when timer is calling the stop button will drop out the energised contactor but it will reinstate as soon as your finger is off the stop button or the stop is released if latch type.

You have a few options like selecting a timer that gives a pulse single only at time on command so it mimics a start button option and does not remain closed once pulsed so will operate within requirements... other options either get a bit more complicated and/or expensive.

As you can see it can be simple to put the timer contacts across the start button but unless you understand and are knowledge in the requirements of control systems and the workings of them under both normal and abnormal conditions then you could end up with an accident waiting to happen that no insurance will cover you for.