Contactor control advise

[Dan smith]

hi guys could you help me out please.

I have been asked to put a contactor on a 100 amp supply to a dB on a food stand. The contactor needs to shut off on fire through a firetrace system already installed.

1st I was thinking of coming of the dB with a 2a breaker to feed the pressure switch them back from this to pull my contactor in but I have just realised I can’t do that as the dB will be dead until I get a feed to pull my contactor in.

Can I come off the incoming L1 to a 2a breaker if so will I have to double up with the same size cable 25mm in the contactor or can I use smaller cable??

Thanks in advance.

 

[Darkwood]

You can get switchgear which will do the same job although more expensive that monitor a closed loop, opening the loop would trip the main switch, what make and model is the board you may be able to upgrade it.
I myself do machine control and their safety systems but I do not know regulations surrounding the request you have been ask to do, it may be the fact you cannot simply use a contactor to do this task, you may need to design a more fail-safe system, if the contactor jams in what are the safety aspects of this in a actual fire scenario.

PS - if you have been requested to fit a contactor it may be the case they are also not clear on requirements but that would then be down to you to explain them and choose a suitable set up.

 

[Dan smith]

The boards is Schneider so I have ordered a 100a 4 pole n/0 contactor as the client asked for n/o. I understand what you are saying if the contactor sticks in but I am not sure of the regulations on this.

 

[Darkwood]

I would heed caution to ensure the cart is not leading the horse here, if you are doing this work it is down to you to ensure it meets safety regulations, is your client asking because they have been instructed by say building control or the local fire safety authority?
I would first find out who has requested the addition and why, then I would look into the relevant regulations it would fall under, the system you propose fitting is inherently not fail safe so I would move forward with caution as fitting this solely on a bare request.

Schneider do do switch gear with integral trip systems which may of better suited your needs but I still would have to ensure they meet the relevant standards for use in safety critical applications.

The last system I saw like this had both systems working together so the failure of one wouldn't stop the disconnection, it also controlled by a safety relay which would ensure reset failure if certain conditions were not met but this did have a few remote E-stop's in the design.

 

[Dan smith]

Firetrace the company who have installed the fire detection pipe around the kitchen have asked for the n/o contactor. It’s been fit as the shopping center it’s in have asked for it.

I get what your saying about the contactor sticking in but they are not designed to do that also how this fire detection works soon as a fire starts the pipe will melt which in turn the pressureused fire extinguishers open and put the fire out immediately. This pipe also going inside the dB and will go inside the contactor enclosure.

 

[netblindpaul]

Just be careful as you are designing a safety system now which is outside the remit of BS 7671, and the consequences of failure could be considerable.
Make sure you are insured to do this, and make sure that the contactor whilst 100A rated has a suitable characteristic for the load.
Darkwood is right with what he has said.
Just remember that there are other ways of achieving this and you really need to consider if the "shopping center" are suitably competent to be defining safety systems.
What they probably require is automatic disconnection of the supply in the event of an actuation of the Firetrace system, and again as DW has said there are other ways of achieving this, which may be more reliable and suitable than a contactor in a box.

 

[Spoon]

Why not look at using a Shunt Trip?
As above, your name will be against this work and the design. Don't let the customer tell you what to buy.

 

[LankyWill]

I’m just being nosey and thinking out loud but can’t you use a fire alarm interface to activate a shunt trip?

 

[Darkwood]

@Dan smith

'I get what your saying about the contactor sticking in but they are not designed to do that'

If I read this correctly you are missing the whole concept of the term fail safe and also not appreciating the fact that contactors do commonly get stuck in the mechanically closed position, this risk increases with heavier loads and constant energised states with which your proposed system would be, a simple thing as a loose terminal can warp the plastics of the contactor and cause it to operationally fail, many other causes too can lead to failure, in your case if it fails in the closed position then this is a safety matter and thus we advice this alone is not a suitable method regardless of you been requested to fit one, you are the professional here so the burden lies on you to comply with the relevant regulations and advise the customer correctly.

I spent 30yrs changing contactors, if you had a small portion of my experience with contactors then you wouldn't use one on its own in a safety critical system.

I have just designed a Cat 3 safety control system to fit upfront on to existing lathe controls, it uses several methods of redundancy like 2 contactors in series, auxiliary monitoring of said contactors to ensure the system cannot be reset if either contactor is stuck in after E-Stop activation, it also uses dual channel and safety relay so all possible failures are covered by a secondary system, the regulation are extremely strict and risk assessment need to be done, so from experience I make my concerns to you that simply fitting a contactor because you were asked probably will not comply and would lay all the responsibility on your shoulders, judges tend not to sympathise with ignorance in H&S cases.

 

[netblindpaul]

I hope that you are following best practice and getting the 13849-2 validation done by a 3rd party DW?

 

[KarPettit]

I have seen many installations where a kitchen distribution board is supplied through a contactor whose control circuit is operated by emergency knock off buttons. As stated previously the n/o configuration will ensure fail safe. with regard the feed to the control circuit breaker, I would assume a rating of 2 or 4 Amps take this from the live side of the contactor if memory serves me right from my panel building days we would use 6.0mm double insulated cable from busbars to feed control circuits

 

[Pete999]

I have seen many installations where a kitchen distribution board is supplied through a contactor whose control circuit is operated by emergency knock off buttons. As stated previously the n/o configuration will ensure fail safe. with regard the feed to the control circuit breaker, I would assume a rating of 2 or 4 Amps take this from the live side of the contactor if memory serves me right from my panel building days we would use 6.0mm double insulated cable from busbars to feed control circuits

Can you explain further and what is doubles insulated cable? do you mean cable with mech protection? and why?

 

[KarPettit]

We used to use double insulated cable as in domestic mains tails assuming a short run within the confines of the contacor housing there is no real risk of damage causing a fault current and any load will be protected by the circuit breaker

 

[Pete999]

We used to use double insulated cable as in domestic mains tails assuming a short run within the confines of the contacor housing there is no real risk of damage causing a fault current and any load will be protected by the circuit breaker

That cable is not double insulated, it's insulated and sheathed