Ring circuit and emergency stop buttons on a contactor

[markymark2406]

Just had a look at fuse board it has the usual 32amp breaker for a ring circuit for a single room and also a 6amp breaker for a contactor for emergency buttons in the same room, there is also a contactor for the emergeny buttons.

This is how i think it works:

the 6amp supply goes into the contactor there it goes to the emergency buttons which are in the closed postion and back to the contactor.

The supply from the 32amp fuse goes to the contactor then to the sockets.

when a emergency button is pressed it opens circuit which takes the contactor our breaking the supply to the ring circuit.


??

 

[ColJack]

that is more of a statement than a question, but yes that's how it works.
the stop buttons shoule be the latching type, but be aware that the sockets will re-energise as soon as you reset the stop button

if it's for something like a school lab then often they require the stop buttons to be key reset types or for there to be a start button to re-energise the sockets.. that way the teachers are in control of it..

 

[ericmark]

If a machine can cause irreversible body damage then the emergency stop system has to be duel line and normally requires a reset button rather than latching stop buttons and often has a special relay which times the difference between both lines and if over so many milliseconds locks out the system so if contacts stick it fails safe.

I have had problems where contactors have been changed for solid state versions and these are not allowed on safety circuits so an emergency stop contactor had to also be added.

The rules and regulations on emergency stops can get complex and one has to do a sort of risk assessment to work out to what lengths one must go. It asks questions like would the person in danger see the danger. i.e. railway track you can see train approaching but some things work so quick you would not have a chance to withdraw in time.

I did have a free book but can't remember who I got it from it was a company who also supplied PLC around Preston area.

 

[bernardgreen]

Absolute minimum is to use a reset button, ( preferable with a key in the school environment ) to re-energise the contactor once it has been de-energised. Mechanical latching E-Stop buttons are not by themselves a safe way to ensure the machinery stays stopped.

The safety system should also take account of risks of malicious interference with the equipment. such as someone wedging the contactor so it cannot open its contacts when de-energised by an E-Stop button being pressed. ( I saw that years ago when a machine operator wanted to delay use of the machine until it was into over time hours. "" Safety system not working, then I cannot use it until it is repaired "" )

 

[Spark123]

If anyone maliciously interferes with a safety system they should be out of the gates before their feet touch the ground - never mind overtime.

 

[bernardgreen]

If anyone maliciously interferes with a safety system they should be out of the gates before their feet touch the ground - never mind overtime.

Agreed but it the days before Maggie Thatcher the unions in two industries held the employers to ransome. One afternoon in a printing works I wrote about 15 service repair dockets for the same machine. As soon as I clear one fault and while writing out the docket another fault appeared.

Fuse blown, easy to spot which one, it was the only one that wasn't dusty.

Door closure micro-switch moved so the door didn't actuate it, All the fixing bolts tight but it had moved.

But as the clock struck 5 the machine ran perfectly.

At least none of those faults made the machine dangerous.

 

[Spark123]

None of the faults you found didn't make it dangerous
What about the ones you didn't find?
If I found an operator (or anyone else for that matter) interfering with anything maliciously I wouldn't think twice about having them escorted off site.
Luckily for me our operators are good that way. OK, some can break stuff by just walking past it but I put that down to the negative energy aura that surrounds them

 

[bernardgreen]

None of the faults you found didn't make it dangerous
What about the ones you didn't find?

The machine was a very well designed printing press and most of it was electrically fail safe.

To have escorted the machine minder off site would have stopped all work for the next few days. So the company had little option but to play the game and cover the costs in the prices they charged their customers.

Most times one would fix the fault and drive off back to the office only to be told when back at the office that it was necessary to go back as another fault had happened. ( before mobile phones ). Only that day I was happy to sit and wait for the next fault, I think because it was a hot day and three hours driving in a van did not appeal to me.

 

[Spark123]

It is only as fail safe as the designer intended and I doubt the took malicious interference into account when designing it.
As for him being the only operator on site then I'd introduce him to his replacement first

 

[vsynth]

A safety system design would never take into account malicious interference, the cost of designing in any foreseeable manipulation of a system would be unacceptable.

How do you stop someone jamming in a contactor or shorting it out with wires?

Overriding a safety system like this is a totally separate matter and one for the courts and HSE.

For the OP I would suggest a system using a 2 channel safety relay e.g. Pilz, and a contactor to switch power to the sockets.

An auxilliary contact on the contactor allows the safety relay to monitor the contactors operation (redundancy) and to fail safe when it fails to operate correctly.

Each emergency stop button has 2 contacts with are both wired to the system so that if one fails the safety relay de-energises the contactor.

 

[bernardgreen]

A safety system design would never take into account malicious interference, the cost of designing in any foreseeable manipulation of a system would be unacceptable.

Oh yes it would, I accept that to take account and prevent every conceivable malicious act could make the price too high compared with the alternative of paying insurance to cover costs of damage and compensation in the un-likely event of malicious actions not forseen at design.

Jammed contacts can be tested by intentional operating as part of routine testing and verifying they did change state.

 

[bernardgreen]

As for him being the only operator on site then I'd introduce him to his replacement first

That was time when the unions operated the "one out all out rule". When registered dock workers had to be employed inland to "unload" the contents of those new fangled "containers" that were being landed at the docks and taken to factories to be un-loaded.

RANT OVER.......

 

[Spark123]

Failure of a single part can be built into safety systems i.e. redundancy.
Me shorting out the lot can't really be built in tho (not that I would without the necessary authorisations but I could.)

 

[Spark123]

As for him being the only operator on site then I'd introduce him to his replacement first

That was time when the unions operated the "one out all out rule". When registered dock workers had to be employed inland to "unload" the contents of those new fangled "containers" that were being landed at the docks and taken to factories to be un-loaded.

RANT OVER.......

I can't even see the unions justifying someone maliciously interfering with safety equipment nowadays.
They may represent them tho.

 

[ban-all-sheds]

That was time when the unions operated the "one out all out rule". When registered dock workers had to be employed inland to "unload" the contents of those new fangled "containers" that were being landed at the docks and taken to factories to be un-loaded.

RANT OVER.......

Is that any different, morally, from what MPs have been doing with their expenses?

Or bankers with their bonuses?

Or Fred Goodwin with his pension?