Contactor chatter and burn out?

[ojchurch]

Hi peeps, go easy on me I don't often post on forums.

I'm working on a school at the moment and several areas have emergency stop systems installed. These are designed to kill the local power circuits with multiple EPO switches.

Whenever I've installed similar systems we've used NO contactors with NC EPO switches wired in series feeding the coils. On this job the circuits that had already been installed when I got there were the reverse, with NC contactors and NO switches wired in parallel.

I can see how both systems would work. The reason I thought you aways did it the way I knew was because it is self monitoring for faults and the default state is with the circuits disconnected. This seems pretty important to me for a safety system.

There were two reasons I was given for their system. One was that multiple contactors can 'chatter' causing unspecified problems. Also that the contactors will burn out.

Can anyone advise me what exactly contactor chatter is and how it might affect the system and/or how long these contactors might last? Or if there has been a discussion elsewhere about these issues point me in the right direction? Or finally if someone knows of a better system altogether?

Many thanks in advance.

 

[johnboy6083]

Putting NC e-stops in series with the coil of a NO contactor is the simplest and safest way to achieve fail safe operation of that circuit. I don't see how you could get chatter, if the supply voltage is ok, and uninterrupted.

 

[johnboy6083]

Having NO e stops is a no no, as it fails to danger.

 

[Knobhead]

At the correct voltage a contactor will last for years.

Chatter can be put down to several thing:
Low voltage.
Dirt on the clapper faces.
An obstruction preventing the clappers meeting.
Faulty / damaged shading poles in the clapper face.

As to using NO contacts for an E-Stop circuit, it’s downright dangerous. A dirty contact could prevent the contactor operating. As John said a NC circuit is self checking.

 

[netblindpaul]

How do they re-energise the contactors, if this is by release of the e-stops then the circuit is in breach of the CoP for schools, which is a BS.
There are a whole raft of H&S and BS requirements around central emergency switching systems for schools and colleges in the UK.

 

[ojchurch]

Thanks for the reply.
You don't happen to have any of the BS numbers/document titles or any more links to these do you?

 

[ojchurch]

Many thanks for all the replies, it's much appreciated.
Actually to make matters worse since I brought the issue up they've decided to use the method I described in new areas. Now the school has two methods of achieving the emergency stop system. Hopefully they will see sense and alter it.
Does anyone know any good links to info on codes of practice/standards for schools? I'll try & get it off the main contractor although they probably won't want to show it to me after 'causing trouble'.

 

[snowhead]

N/C safety cct is as stupid as self resetting safety cct.

They need sorting out immmediately.

I've dealt with contactor controlled ccts including safety ccts for around 40 years and never seen such a thing.

 

[ojchurch]

I agree.
Just to clarify about self resetting: in the correct system do you consider local 'key to release' NC emergency switches wired in series feeding NO contactors acceptable?
Cheers.

 

[Engineer54]

As many others here have stated, i too have never seen N/O emg stops.

 

[Knobhead]

The beauty of key release P/B’s is the teacher can tie down the origin of the e-stop.

In this situation I would go for them.

 

[johnboy6083]

As many others here have stated, i too have never seen N/O emg stops.

this week ive been working on an FG wilson generatiors, in which the external and compound e-stops were wired as NO. This has now been altered by the genny supplier, but i was told that they supply them like that as standard. how many of them must be in service with NO e-stops. also the estops input into the controller, and are not wired independanty. Im not too sure about the legislation, but im sure a safety conroller/plc has to be marked as such.

 

[Engineer54]

We have used F.G. Wilson generators on a few projects, mostly for MV (13.8KV) units. Can't say that i've ever seen N/O em stops, on any of their submitted proposal drawings. That i would have picked up on, ...and changed!! ...lol!!!

 

[netblindpaul]

I agree.
Just to clarify about self resetting: in the correct system do you consider local 'key to release' NC emergency switches wired in series feeding NO contactors acceptable?
Cheers.

Yes as long as releasing the keyed stop button does not re-energise the circuit.
Else it is self resetting.
Another thing is that it is not allowed to have ANY safety related devices connected to a central switching system.
That is no LEV, no dc braking, no flammable gas controls etc.

 

[netblindpaul]

I have seen and used NO emergency stops.

































As long as you have an NC with them!
The NC is hard wired chain the NO just the PLC messaging system!