Emergency Stop Relay

[GGSLE1]

Currently in a project lab in the university I am assisting students with engineering problems. We have a new installation of a emergency stop circuit utilizing a mechanically latched relay for all the wall and equipment power. We have several instances were it has failed to unlatch when the e-stop button was engaged. The relay will be activated as soon as the e-stop is pulled back out without additional action. My understanding is that it should be a magentically held relay. I was informed that Indiana has not accepted NFPA79 as a standard. A reply from an outside engineering firm stated that the circuit with a magnetically held contacts would be safer but a latching relay being used as a MCEmergency disconnect was acceptable. Any thoughts on this?

 

[MikeHalloran]

Indiana doesn't accept Daylight Savings Time either.
That's only germane if you're in Indiana, a fact not in evidence.

You must not be a regular teacher; it usually takes them two or three single spaced pages of text just to say hello. You are brief enough to possibly be an engineer.

You will probably be able to lead yourself to an answer to whatever your question is, by being a little more wordy in exposition of your situation.

I.e., verbally link the limits of your problem space, and the locus of the solution space should become apparent. Answer the unasked questions that become obvious.


Oh. Magnetically latched relays are a product of The Devil.




Mike Halloran
Pembroke Pines, FL, USA

 

[dbaird]

A risk assessment must be done and the results of that will tell you the risk category. The risk category determines which type of safety relay is required.

David Baird
Sr Controls Designer
EET degree.

http://www.afctool.com

 

[controlsdude]

If you depending on a estop relay to isolate power your using the wrong device for this purpose. Disconnects with locks are the method. Estop relays are only used to stop machinery in case of a safety incident.

 

[ScottyUK]

I am pretty sure your second sentence describes the situation, controlsdude.

I have no idea what the rules in Indiana might be, but for this type of application in the IEC world the basic premise is that the scheme should fail to a safe state, whether that failure be a power outage, cable break, and so on. In that regard, either a magnetically-held contactor or a circuit breaker equipped with an under-volt release would both satisfy the requirements of the power switch. A latching contactor is a very odd proposal for a simple safety system such as this, at least to those in the IEC world.

Higher integrity E-stop schemes use (for example) safety relays by the likes of Pilz to control the power switch, resulting in an overall system with a lower probability of a failure resulting in danger.

 

[LionelHutz]

By magnetically latched do you just mean the energized coil will keep the relay closed? this would not be latched at all, it is simply a N/C contact on the E-stop energizing the coil.

I would not use a latching relay. How does that work anyways? The E-Stop energizes some kind of trip coil to drop the relay out.

If safety is very important then a simple E-stop with a N/C contact is no good either. The contact can fall off the operator and then it just stays closed.

 

[ScottyUK]

Lionel,

Magnetically latched relays need a pulse to close and a pulse to open. They have their place, but in my opinion a safety circuit like this isn't one of them. They are more common in large sizes with high power coils.

 

[LionelHutz]

I meant to wrote mechanically held.

 

[genebond]

Mike:
Indiana (re)adopted daylight savings time a few years ago, since the rest of the US wasn't smart enough to drop it like we did in the 60's, or to remember that Indiana and Arizona brightened up and quit changing their mind about what time it is

As to the application, there are several ways in which to set up an E-Stop circuit, most of the decision is dictated by your insurance carrier and their lawyers. From there, it's personal/professional opinion. Historically, a electrical latching cu=ircuit has been adequate for general applications, or for a more sensitive area, use a designated safety device, such as an E-Stop safety relay, which is guaranteed to fail in a safe state. Some systems use a contactor to kill all power, while others use shunt trips on circuit breakers to do the same, and yet others just kill control voltage. YMMV