emergency stops

[WELLSIANA]

i am only just starting out on commercial work after 35 years in domestic.
I have to fit emergency stop buttons to some machines..most are just at the moment on plug tops like small grinders with a on/off toggle switch on the machine..and i allowed to just fit a E-STOP with key to the flex or do i have to blank the toggle switch and fit in line stop/start button...plus 3 machines are really old 3 phase with a stop/start fitted internal what would be the best and cheapest way to fit a E stop to a machine with its own current stop/start 3 phase..any help really appriciated as simple as possible due to my lack of commercial knowledge as its a sector i have to expand into now due to climate...thanks

 

[bernardgreen]

Unless you have the ability and knowledge to undertake a proper risk analysis of the location and machinery in use do NOT attempt this work.

For some machines simply removing all power will likely make the accident worse if the machine needs power to shut down safely. Work pieces held in place by magnetic clamps may fall or be thrown from the machine if there is a sudden power cut from an ES being operated.

An Emergency Stop in a flexible lead is a definate NO

The ES switch has to be located in clear view and at a fixed location so that it is easy to reach by anyone.

For hand tools in a workshop it may be best if the power to all the sockets that are used for hand tools is switched off. The reason for this is that the operator in danger may already be injured and hence not be able to hit the ES button. Someone elsewhere who sees the accident will be the one to hit the ES button so in a workshop it may be necessary to have more than one ES button

 

[GaryMo]

With the grinders it would be best to either remove the toggle switch or just leave in the on position then install a direct on-line starter with integrated e/stop and fix to the front on the grinder.

With the older machines, surely you can get to the internal wiring of the existing contactors/overloads and route from there to an external e/stop?

 

[ericmark]

LC Automation did do a very good free book on E-Stops. As already stated it starts with a risk assessment and were irreversible damage to the body will result if the E-stop fails normally it needs everything in duplicate.

So an E-stop in just the control circuit would likely not comply. Often a second contactor is also required. Normally duel contacts on the E-stop some times one N/O and one N/C going to a special relay.

As already said in some cases so parts need to be kept live for example supply to motor brake so one has to be very careful. Also remember if has to act on all power which includes pneumatic. That really causes a problem as not only does the air need switching off it also often needs dumping as well. This can cause rams to drop and doors to open I had one where hitting the E-stop caused a ton of concrete to be dumped on the floor. I had to split the air feeds with one being switched off and dumped but the other was maintained.

Items like grinders again clearly need a risk assessment. My problem with grinders as supplied after a power outage they would restart without pressing any reset. I cured by fitting active RCD's to the circuit as grinders were not very big. But I did not do risk assessment we had a safety officer who had a list of letters behind his name to decide what was acceptable.

As electrician in fact even when employed as electrical engineer always there has been a safety officer who would decide what should be done. As electrician I would suggest a method or maybe even a couple of methods but it was never up to me to decide if the HSE would likely accept what I had suggested always some one with a line of letters behind his name would have the final say as to what was done.

Some times one can without realising it fall foul I did this with come contactors which were regularly failing due to frequency of use and I changed them to solid state forgetting about the E-stop regulations. I then had to fit extra contactors and e-stop relays to bring it back in line with the regs.

 

[WELLSIANA]

thank you for your interesting replys to date....heres how i was going to go ahead and welcome your views.
the grinders are plugged in 13 amp i was going to omit the toggle switch and fit a direct online starter to each with a emergency stop beside in a n/c contact so to hit it pops out the starter too which needs to be manually reset.
The bigger machines all have stop/start switches fitted to them and some have an old emergency stop built in without mushroom or colour..i was simply going to mount beside the present one and divert it through my e stop again n/c..like fitting a clear e stop beside a old time one or i could even get rid of the old one and blank off and fit the new one...thats what i had thought of doing but i will admit this is pre inspection and client based only to prepare for an inspection if i am better to wait then i could advise but looking at my suggestions does anyone see it not being ok ..again thanks

 

[bernardgreen]

the grinders are plugged in 13 amp i was going to omit the toggle switch and fit a direct online starter to each with a emergency stop beside in a n/c contact

Make sure the grinder cannot be unlpugged and then plugged into another socket that is not ES protected. The operator has an accident, screams, someone hits the grinder's ES but as the grinder is not in it's allocated ES protected socket it keeps running ( maybe grinding the operator ). The grinder is now a hazard that delays rescue of the injured person(s).

There is a hazard that other workers may put tools down to go to help their colleague without ensuring their tool is safe. An ES power down to all hand tool sockets reduces that hazard.

It would make things safer if ALL hand tools had switches on the tool that released to OFF when power was removed.

 

[ericmark]

Size matters. The rotating mass of a grinder means after switch off it will continue to run for some time. Many methods are used to arrest the wheel including eddy current breaking and one has to be very careful not to remove a safety feature in ones attempt to add one.

In the main E-stops on items like grinders are to allow the operator to stop it without using their hands. Common to have a knee hight E-stop.

As a wheel slows it can grab the item being ground and again I would be saying I am the electrician not the safety officer you tell me what you want and I will do it but I have not got the expertise required to work out what is required.

Remember with any safety work everything must be in writing. Clearly some one has done a risk assessment you should ask to see that risk assessment first. Verbal instructions are not acceptable with health and safety items.

 

[GaryMo]

the grinders are plugged in 13 amp i was going to omit the toggle switch and fit a direct online starter to each with a emergency stop beside in a n/c contact

Make sure the grinder cannot be unlpugged and then plugged into another socket that is not ES protected. The operator has an accident, screams, someone hits the grinder's ES but as the grinder is not in it's allocated ES protected socket it keeps running ( maybe grinding the operator ). The grinder is now a hazard that delays rescue of the injured person(s).

Don't install the DOL starter and e/stop prior to the socket, install them on the pedestal grinder so the flex supplies the contactor and the grinder is supplied from the overloads.

 

[WELLSIANA]

the grinders will each be fitted into the dol starters with so will not be able to be unplugged.

How do we feel about the other ones that have a stop/start and sometimes a e/stop old type fitted.
surely to just add a new yellow mushroom head beside the on present does not actually make it any different to how it is at the moment except with new complient markings.
I will admit at present i have client only instructions but we also can cloud issues so also what would any of you do if having the same instructions.

 

[GaryMo]

Sounds fine to me. You've not carried out a risk assessment nor brought the machinery up to a safety category which was dictated by a risk assessment. All you're doing is installing some DOL starters and e/stops, some of which are just additions to existing. Already the machinery would be more compliant with PUWER98 than it was previously. By carrying out this work you're not making the machinery fully compliant with PUWER98, just make sure the client is aware of that, ideally in writing.

 

[bernardgreen]

I will admit at present i have client only instructions but we also can cloud issues so also what would any of you do if having the same instructions.

I would walk away from it. Only if there was a WRITTEN authorisation from an APPROVED health and safety person would I act on the client's instructions.

As Eric mentioned some rotating machines use various means to slow the rotation and one method is to remove the power and then short the motor terminals so the motor acts as a generator into a dead short ( or resistor ) disipating its rotational ( inertial ) energy as heat. Just cutting power may not put the short ( or resistor ) in place the the wheel will continue to rotate much longer than when it is turned off normally.

Another danger is slowing a large rotating mass too quickly by slamming on an electrically released brake which may happpen if power is removed in the wrong way. Either the wheel shatters or the axle and brake mounting breaks and the rotating mass falls out of the machine.

 

[GaryMo]

Are we not talking about simple pedestal grinders here?!

Regarding the other machinery, he's going to install an e/stop into the same control wiring as the current e/stops.

I think most of you are going way OTT!

 

[WELLSIANA]

i appriciate all help but agree yes these are simple 4 amp area bench grinders like you have at home ..they will not pull you in too much , fingers maybe..the only thing i could do without is 3 grinders all together but need 3 starters as each may be used at different times.
they are on the same ring so i am thinking is it cheaper to break power to the ring via a contactor than 3 starters and e stops...if anyone could do a diagram of how you break the ring like that i would again appriciate it as in all cases the e stops are rated at 10 amps and obviously the rings 32amps...if you feel that would be cheaper a sketch would really help ..thanks

 

[GaryMo]

I'd stick to independent DOL starters with e/stops mounted on or by each grinder.

 

[stillp]

What's the purpose of an e-stop on a bench grinder? They can't be stopped quickly without a risk of bursting the wheel.