Class room emergency disconection

[Wontdothatagain]

Am planning for a job in the new year. It is for a small wood working training room. There will be a mix of 110v & 230 sockets. Customer has asked that all sockets are on 10 emergency stop, lockable buttons ie the ones where you have to insert a key to release them and the same for each of the machines.

I was thinking of putting the sockets through a contactor and the emergency stops to energise the contactor coil but am not sure about the machines as there are 26 of them and i don't want to fit 26 contactors.

Any suggestions.

 

[wingcoax]

Find out the max load of the machines and fit heavy duty contactor/s as required. series up the p/b's and bob's your watsit.

 

[crystal ball]

you can buy off the shelf units for this situation, have a look at Enbray contactors

 

[tonyelectric]

My firm specialises in school workshops and servicing the machines therein and what your client has asked you for is wrong, it won't comply with the BS for D&T in schools.

The approved set up is to have NC contact emergency stop buttons wired to a contactor that will latch off and therefore can only be reset from one key point. The use of multiple resets is not permitted because it would be possible to reset locally and possibly re-start something where an accident had occoured in another area of the workshop. often schools also use the system to shut off the power in breaks. Generally the power to the contactor coil is wired in parallel to a red bulkhead light in the corridor above the workshop door to advise that the power to the machinery is on.

We use custom built contactor units that also contain an RCD and outgoing MCBs so giving an all in one unit.

 

[tonyelectric]

wiring methods and equipment need to comply with 17th ed AND
BS 4163:2007 Health and safety for design and technology in schools and similar establishments. Code of practice

 

[bernardgreen]

Effective emergency stop for woodworking machinery is not always achieved by simply removing power to the machine.

Removing power may allow the machine to coast to a halt rather then being bought to a halt in a rapid but safe way.

Coasting to halt is not a problem if there is a power failure but coasting to a halt in an emergency stop may not be acceptable when injury is happening.

Tjhe client needs to do a proper risk assessment on the machines and how they are controlled during an emergency stop.

 

[tonyelectric]

Bernard makes some good points, machines must come to rest within 10 seconds and if not be retrofitted with regenerative breaking units, certain machines must have a key switch fixed locally on the machine and this must be of the type where the key cannot be removed in the on position. Many machines require additional foward facing stop controls or floor stops fitted. Wiring on and to machines from wall points must be in SY. Machines need limit switches fitted to prevent doors being opened etc. And certain machines are not permitted in schools.

We always advise schools to buy educational versions of machines because the cost of upgrading non-educational equipment can be very high as there is often both mechanical and electrical work to be undertaken. I have two pillar drills in the workshop to be modified for a school, the guards that comply with BS4163 cost £130 to me, so a saving has just turned into an expense for them.

Axminster, Wadkin & Boxford all sell school versions of their kit that are specced up to plug and play.

Schools get audited on their workshop kit by the counties appointed mechanical engineers/ heath and safety advisor once a year, I spend a good few months in schools correcting errors in workshop kit and it costs schools a packet.

If you need guidance i'll be happy to answer any questions, feel free to PM me if you wish.

 

[Adam_151]

Could consider fitting a separate DB for the stuff on the stop button and have the contactor drop the power to the whole DB rather than a contactor per machine/circuit if theres lots

And like tony says, make sure the reset button requires a key to reset it (to prevent kids from powering up the workshop), thats as well as stop buttons which latch in and need to be pulled out before you can reset

Watch out for stuff that uses electronic braking though (I'm sure eric will come along and add something...but you have to get the controller to brake the machine if the stop is pressed, rather than just cutting the power as otherwise it would just spin down slowly)

 

[tonyelectric]

Its worth running your scheme by DATA (design & technology Association) they co-author the BS4163 document and are really helpful and will willingly look at your ideas to ensure compliance.

Incidently you don't need a wall stop per machine, as a rule one by every door and on every wall is sufficient, we put a hinged perspex cover over them to prevent kids mucking about (these covers are available commercially)

The breaking units we use don't require power so even once the power is cut the machine breaks, the dust extraction and ring circuit should cut too leaving just lights on (these generally come from a different supply altogether)

 

[Wontdothatagain]

My thinking was,

DB1 to have all non critacal items ie lighting

DB2 to have a shunt on the main switch

All emergency stops are to be N/C key types so that the teacher would have to unlock the system if one of the stops was pushed and this circuit would control the shunt.

Are you saying that equipment in schools HAS to have breaking units on rotating machinary even if it is not supplied from the factory?????

 

[tonyelectric]

On certain machines yes, bandsaws, planers and circular saws are required to have breaking even if not factory fitted, if the machine were not built to school spec there will be quite a lot of modifications to make to bring all the guarding up to spec unless you are lucky.

Your idea with a chain of NC stops with keys back to a central contactor may scrape through, the issue will be that the county engineer will argue (and i've had this discussion) that a child could obtain a key that has been left in the switch and use it to restart. We usually fit the reset key switch in the prep room.

Two DBs is fine, but as only lights are left after a stop if they are fed from a DB say in the nearest corridor then a second DB is redundant.

All council schools will get an inspection once a year from one of two firms who are appointed to inspect, if they can find anything wrong they will and charge the school £££ to put right. For example an electric fretsaw with no volt release switch factory fitted on the side must have that switch removed and a separate starter fitted with a front facing dome stop or floor stop, the charge by one of these co's to do the work is never less than £400 and the councils hang on their every word. If they find you've used a bit of plastic flexible conduit they will have you publicly flogged

 

[ColJack]

you don't fit 26 contactors.. you fit one big one and feed a DB off it..

unless you mean you want seperate e-stops for each machine.. but it doesn't read like that..

 

[Lectrician]

Yup, fit two DB's - one controlled by a contactor.

 

[Wontdothatagain]

So, would you all fit a contactor "before" the main switch or a shunt combined in the main switch????????????

 

[tonyelectric]

I do it like this


Main switch (MEM excel lever type)
--
--
Contactor
--
DB (RCBos)
--
Local isolators per machine
--
commando socket plug
--
Machine