The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

[tgstoll]

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

2011 NEC Article 700.5 (C) requires all transfer switches to employ contacts that are "electrically operated, mechanically held".

Please define, once and for all, what, exactly, "electrically operated, mechanically held" means. I am sure this question has been asked before, but I believe there is still some confusion about the accepted definition.

Also, a quick question about this topic:

Would a simple Form C relay (appropriately rated) used in such a way as to connect, for example, a 20-amp branch circuit lighting load to the emergency circuit using the relay's NC contact that is "mechanically held" with spring tension (relay coil de-energized since normal AC line power has been interrupted) and reconnect the normal circuit to the lighting load when the relay coil is re-energized (electrically operated) when normal power resumes?

Thank you for your consideration, review and response to this forum request/question. :thumbsup:

 

[templdl]

In my view point electrically operated is simply something that can be operated by PB or remotely W/O and mechanical operation by a human.
Mechanically held would mean that after the device is closed electrically it no longer needs and electrical signal/power to stay closed such as keeping a coil energize as in electrically held.
The common operation of mechanically held would be if the relay or contactor were held in the closed position by a magnet (it could be a mechanical latch also).
To release the "magnetic latch" an electric current (pulse) is induced it mess up the magnetic hold of the magnet which then releases the contactor or relay.

Should power be lost the relay or contactor wold not drop out unless there was a separate source poweravailable that could be used to unlatch them and drop them out.

 

[tgstoll]

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

I understand your response. Thank you. What you provided, though, is an example, not a defintion. Can you provide a general definition that is accepted by the NFPA 70 (NEC) and UL 1008 (Transfer Switch standard) groups? Also, would you say the example I gave comply with the intent of "Electrically Operated, Mechanically Held" as it is called out in the NEC and UL 1008? Thank you again for your repsonse.

 

[zbang]

A "mechanically held" device is one that will stay in whatever position you put it without external energy (like a 3-way wall switch). Most contactors and relays can not be called "mechanically held", since when the external energy goes away, they may change state.

 

[tgstoll]

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

Thank you for your response.

Again, a clear definition is what we are missing and is yet to be provided. This is why there is confusion about what the "Electrically Operated, Mechanically Held" requirement really means. Can anyone provide this definition? Once this has been clearly defined, it will be easy to determine whether or not a device or scheme complies and will, hence, answer my question.

 

[GeorgeB]

I thought zbang's answer was complete, but I'll try to reword it ... not improve it.

An "electrically operated, mechanically held" device is one that will stay in whatever position you put it electrically without maintenance of the electrical level.

I'm in the control of fluid power field ... a 2 position solenoid operated detented valve is relatively common in pneumatics, rare but exists in hydraulics ... exactly what I see as the definition.

 

[tgstoll]

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

OK, that gives a clearer definition. Thanks!

As defined, "electrically operated, mechanically held" means a contact exhibits this property when it will remain in whatever position you engage it electrically without maintenance of the electrical control signal required to activiate it.

Now, one more clarification. Assuming this is our definition, how does the example I gave of a Form C relay where the emergency position is in the de-energized state and is mechanically held in this position with spring tension NOT comply? The relay is only electrically operated in the normal condition, but defaults to the emergency condition when de-energized. The emergency condition would be "electrically operated" by removing the electrical signal (the loss of the AC line) and is "mechanically held" with the spring tension. Why would this not comply, or does it?

 

[david luchini]

Now, one more clarification. Assuming this is our definition, how does the example I gave of a Form C relay where the emergency position is in the de-energized state and is mechanically held in this position with spring tension NOT comply? The relay is only electrically operated in the normal condition, but defaults to the emergency condition when de-energized. The emergency condition would be "electrically operated" by removing the electrical signal (the loss of the AC line) and is "mechanically held" with the spring tension. Why would this not comply, or does it?

Your relay is not "mechanically held", it is "electrically held." Starting with everything denergized, you apply power to the coil and the relay changes state (lets say closed.) You remove power to the coil, and the relay changes back to its original state (lets say open.) It was being held in the closed state "electrically." If it had stayed closed after power was removed from the coil, it would be "mechanically held" in the closed position.

 

[zbang]

As defined, "electrically operated, mechanically held" means a contact exhibits this property when it will remain in whatever position you engage it electrically without maintenance of the electrical control signal required to activiate it.

Sort of... don't think of contacts, think of the entire device. Does the device maintain state when an activation signal is removed? Also, don't think of just two states, there may be more (as in a rotary switch, where you might have up and down pulses to change the connections).

Assuming this is our definition, how does the example I gave of a Form C relay where the emergency position is in the de-energized state and is mechanically held in this position with spring tension NOT comply? The relay is only electrically operated in the normal condition, but defaults to the emergency condition when de-energized. The emergency condition would be "electrically operated" by removing the electrical signal (the loss of the AC line) and is "mechanically held" with the spring tension. Why would this not comply, or does it?

It does not comply because when the electrical signal that puts it in the "normal" position is removed, the position changes. It is not mechanically held in all positions. "Electrically operated" generally means supplying power to change the state, not removing power.

 

[tgstoll]

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

OK, this makes sense. I have one more follow-up question.

Does the "electrically operated, mechanically held" requirement for a transfer switch, specifically, apply to both the emergency and the normal condition or just the emergency condition, when power must be present to energize the emergency lighting load?

 

[iwire]

OK, this makes sense. I have one more follow-up question.

Does the "electrically operated, mechanically held" requirement for a transfer switch, specifically, apply to both the emergency and the normal condition or just the emergency condition, when power must be present to energize the emergency lighting load?

Yes it applies to both conditions.

But it is a moot point, you are required to use transfer equipment, identified for emergency use, a typical relay will not be.

700.5 Transfer Equipment.
(A) General. Transfer equipment, including automatic transfer
switches, shall be automatic, identified for emergency use,
and approved by the authority having jurisdiction...........

 

[tgstoll]

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

The End-All-Be-All Final Definition of "Electrically Operated, Mechanically Held"

OK, now all my questions have been answered except one.

If the Form C relay is a latching type for both the NC and NO positions (electrically operated to transition from one position to the other, but is latched, or mechanically held, after engagement) and is rated for emergency use per UL 1008, then could it be used for emergency purposes as a "transfer switch"?

 

[templdl]

OK, this makes sense. I have one more follow-up question.

Does the "electrically operated, mechanically held" requirement for a transfer switch, specifically, apply to both the emergency and the normal condition or just the emergency condition, when power must be present to energize the emergency lighting load?

Do you have a particular manufacturer and product in mind that you are trying to defend or is this a theoretical question? I'm sure we could evaluate a product regarding how it functions.
I appears as though we may be shooting at shadows.

 

[templdl]

I found this explaination:
An electrical switching device having an actuator reciprocable between two extreme positions, the switch being open and closed, respectively, when the actuator is in its two extreme positions. An electrical operator, such as a solenoid having a movable armature, responds to successive momentary energizations by moving the actuator first to one extreme position and then the other. The actuator is mechanically held in each extreme position by a spring-biased element which also serves to transmit movement of the actuator to the switch contacts. The actuator may have a cam surface shaped to cause opening and closing of the switch contacts, the cam surface also cooperating with the spring-biased element for urging the actuator toward each of its extreme positions. The switching device may be used for interrupting electric power to a customer of a utility by being located between a watt-hour meter and the customer's load, or in other similar applications.

 

[texie]

OK, now all my questions have been answered except one.

If the Form C relay is a latching type for both the NC and NO positions (electrically operated to transition from one position to the other, but is latched, or mechanically held, after engagement) and is rated for emergency use per UL 1008, then could it be used for emergency purposes as a "transfer switch"?

No, as Iwire stated the point is moot as there is a lot more to a xferswitch rated (identified) for art 700 use than just being mechanically held. Now if this was an optional standby deal I think my answer might be different.