Mechanically-held contactor for Emergency Stop circuit 2

[zzap]

I am working on renovations to a High School machine shop which involves relocating the shop from one room to another. The idea is to reuse equipment where possible.

The existing shop has rotating machines fed from a panelboard which is fed from a lighting contactor. The coil of the contactor is wired to 3 emergency stop push buttons around the room so that the activation of any of the 3 buttons will open the contactor, killing power to all the machines. In the existing system, the contactor is of the mechanically-held type: the e-stops open the contactor, a keyed momentary-action switch recloses it.

My problem with this system is that in the event of a power outage with the contactor in the normally-operating closed state, when power is restored many of the machines will come on automatically. A few of the machines have contactors built in, others latch on/off. I feel this presents an infrequent but serious hazard (keep in mind we're talking about a high-school environment).

I see three options:
1) Status quo. Warn the shop instructor about restart.
2) Replace the existing contactor with a (fail-open) electrically-held contactor (complete with a momentary-action start/reset switch and seal-in contact).
3) Add local contactors with start/stop momentary-action PBs to machines which don't already have them.

I have been advised by a senior PEng (I am an E.I.T) that the existing equipment is sufficient and the budget is small (contactor replacement would be 5-10% of electrical construction budget). There's politics involved, but I would like to have some second opinions on what is the best engineering solution.

Your comments would be greatly appreciated.

 

[davidbeach]

I think that the best way of doing this type of system is to use a main breaker in the panel with an undervoltage release on the breaker. Run the undervoltage release through NC contacts on the EPO switches. On loss of voltage to the undervoltage release, either from loss of power to the system or from an EPO being pressed, the breaker will open and can be manually reclosed once voltage is again available at the release. The control circuit will have to be tapped from ahead of the main breaker.

 

[rbulsara]

I agree with davidbeach..in the apporach and the use of undervoltage which is a better way to EPO a machine shop.

A shunt trip scheme requires power to operate and if a wire is broken it may not work when needed.

 

[itsmoked]

Status quo, utterly unacceptable.

 

[RonShap]

davidbeach,
Great idea. Could you give a suggestion for a low voltage CB, maybe 100A or so, that has an undervoltage release, that doesn't require external power to function (other than the voltage that is being sensed, as it may not be there to activate the trip). I'm not aware of a breaker that will trip on its own on undervoltage without some other type of circuit to activate a shunt trip.

 

[Skogsgurra]

Hi zzap,

It is good that you react to this kind of really dangerous installation. I would do what you are saying in your #3 item on the list. Eliminating the risk that someone loses a finger or a hand just once should not be a question of $$$. There are safety codes in most countries (do not know where you are) that prescribe that an undervoltage swith (a contactor with push-button on/off) shall be fitted to each single machine with an electric motor.



Gunnar Englund

http://www.gke.org

 

[jraef]

While I agree that a UV release on the main CB would have been the right way to do it up front, your budget constraints may make that unpalatable. Most older CBs in North America cannot have UV releases added in the field, they must be ordered that way from the factory. This now means replacing the main CB, and if replacing the contactor is too expensive, that rules out the CB as well since it is likely more expensive than the contactor.

IMHO the least expensive option would be to add UV releases to the specific machines that do not already have 3 wire control, but that may not mean adding contactors and push buttons to them. When you said they "latch on/off" I am assuming that they have Manual Motor Starters on them. Most of those have UV release coils that can be added in the field very inexpensively, essentially giving you the same protection as 3 wire control on a contactor. If they don't, you can get small inexpensive Motor Protection Switches from a variety of sources such as Siemens, ABB, Moeller, A-B, Square-D / Schneider etc. These can be fitted with UV releases as well.

I totally agree, status quo = unacceptable. It's a tragedy waiting to happen.

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[davidbeach]

RonShap, try page 153 and following of

http://www.eatonelectrical.com/unsecure/cms1/PG74A01TE.PDF.

The other major manufacturers also provide the same.

 

[fsmyth]

How about a relay in the e-stop circuit?
Make it normally activated. Bypass or
disable it manually after power restoration.
<als>

 

[LionelHutz]

Install the e-stops like stop buttons in a 3-wire control and then put another button to "power up" the shop (you already have this it seems). Change the contactor to be a standard type with coil power required to hold it in. We used a contactor in our test bench for many years wired like this and it worked fine.

 

[zzap]

In closing:

Thank you all for the posts.

The client elected to hire his electrical maintenance contractor to do the fix - leaving it out of the Scope of Work for my project. The contractor chose to install starters (3-wire control) on the machines that presented the danger.

As an example, the bench-top grinders have a built in on/off button; I don't think (I'm guessing) that it's possible/feasible to add U/V protection to such a device without having to add an entire 3-wire starter - as was eventually done.

It was also a cheaper solution than the contactor/main CB solutions - though not as fail safe, given the possibility of future additions of machinery.

Thanks again.

 

[ScottyUK]

Glad you got it sorted - I'm slightly surprised that it was cheaper to fit an NVR starter to each machine than to the main service, but it is a good solution.


----------------------------------

One day my ship will come in.
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[jraef]

ScottyUK,
That's because as I mentioned earlier, many older design MCCB's here in N. America did not have the ability to retrofit a UVR coil into them. The breaker had to be removed and sent back to the factory to have it installed because it involved messing with the trip mechanism, and required recalibration in most cases. New (last 10 years or so) breakers have that ability now of course, we learned that from IEC designs. You guys come up with a good idea once in a while.

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