Momentary e-stop and square D mechanically latched contactor

[Caleb907]

Hello, I'm currently working on a school up here in remote Alaska and I need help with how to wire this contactor correctly. All of the receptacle power in the science room is switched thru this contactor (Square D), mechanically held and a H.O.A. The e-stop is mounted next to the door so that a teacher can shutdown all of the power in case of an emergency and it's a momentary push button (NO/NC contacts). So I guess my question is if I wire my e-stop through the auto, where do I land the leg coming back, so that when they push it, it won't latch again as soon as they take their hand off the push button? I guess I don't understand what keeps the relay from changing state as the button is released? I see that there is a terminal block with one terminal labeled(1), the other(2), which is my line voltage and neutral, and according to the schematic there is an (A1) and (A2) which appear to be on the relay. It all appears to be pre wired from the factory. Although it shows a dotted line labeled "two-wire control" thru a NO contact between (A1) and (A2) which I'm guessing is my auto.... (A2) is my neutral off my relay though.... It seems no matter how I wire it, it latches and unlatches when I press the e-stop. I feel like its so simple but it's just not clear to me what the heck I need to do!! Please help, thanks, Caleb

 

[pjholguin]

I have few questions and statements:

1. Why are you using a HOA? What would the auto go to?
2. The e-stop is normally placed between L1 and the first switch or device.
3. A mechanically held contactor requires power to turn and power to turn off...I have seen them used in lighting & HVAC controls
and normally their magnetically held from my experiences...there maybe other types.

PM me if you want discuss this more...PJHolguin

Hello, I'm currently working on a school up here in remote Alaska and I need help with how to wire this contactor correctly. All of the receptacle power in the science room is switched thru this contactor (Square D), mechanically held and a H.O.A. The e-stop is mounted next to the door so that a teacher can shutdown all of the power in case of an emergency and it's a momentary push button (NO/NC contacts). So I guess my question is if I wire my e-stop through the auto, where do I land the leg coming back, so that when they push it, it won't latch again as soon as they take their hand off the push button? I guess I don't understand what keeps the relay from changing state as the button is released? I see that there is a terminal block with one terminal labeled(1), the other(2), which is my line voltage and neutral, and according to the schematic there is an (A1) and (A2) which appear to be on the relay. It all appears to be pre wired from the factory. Although it shows a dotted line labeled "two-wire control" thru a NO contact between (A1) and (A2) which I'm guessing is my auto.... (A2) is my neutral off my relay though.... It seems no matter how I wire it, it latches and unlatches when I press the e-stop. I feel like its so simple but it's just not clear to me what the heck I need to do!! Please help, thanks, Caleb

 

[Smart $]

In order to provide direction, need contactor model number...

 

[Besoeker]

3. A mechanically held contactor requires power to turn and power to turn off...I have seen them used in lighting & HVAC controls
and normally their magnetically held from my experiences...there maybe other types.

Yes. That puzzled me too.
The idea of an emergency stop push button and a mechanically latched contactor seems like incompatible philosophies.
You hit that red button and you want the power to be dropped. No ifs, no buts.

For emergency stop circuits, we use electrically held contactor and latching (stay-put) key release, mushroom head push buttons.
The power can not be restored until the EM stop button has been released by a key holder.

 

[wallypiper]

Although I'm not familiar with them, Siemens, Schneider, and Allen Bradley all have "electrically driven mechanically held" contactors in their catalogs and all show wiring with an HOA switch as an option. All are labeled lighting contactors. I expect you can find a way to e-stop them with a 2 wire control circuit and a momentary contact.

w piper

 

[Besoeker]

All are labeled lighting contactors. I expect you can find a way to e-stop them with a 2 wire control circuit and a momentary contact.

But why would you want to when a standard contactor would do the job?
Is there something I'm missing here perhaps?

 

[fmtjfw]

Who would specify a mechanically latching contactor for this application?

Who would specify a mechanically latching contactor for this application?

If you have a Normally Closed contact on a non-momentary E-Stop pushbutton which supplies power to a "standard" contactor coil which is electrically held (by the magnetism of the coil) then any failure (excluding welded main contacts) will fail safe by cutting off the power. This is the normal way to implement this function.

You could also have a momentary E-Stop with the standard 3-wire start-stop pushbutton circuit, provided you made the Start button not readily accessible. This, however, has several failure modes that would require the Stop pushbutton to be held in (e.g.: short across the Start pushbutton, or welded auxiliary Normally Open contactor contact.)

The problem with a mechanically held contactor as the emergency "turn off power" is that it would need a control pulse from the E-Stop pushbutton to force it open. If the power source to the E-Stop pushbutton fails or the pushbutton wiring fails without the power failing to the line terminals of the contactor, you have lost the ability to cut off the power.

 

[jim dungar]

In my experience, very few of these installations are actually E-stops. Many of the 'Off' buttons are located in restricted area (e.g. shop office), or there is only one for the entire room. Most of these contactors are more in line with the power off circuits we see in data centers.

The primary purpose appears to be to turn off the shop equipment so it cannot be used outside of allowable hours. Yes, emergency stopping is a side benefit.

For a mechanically held contactor, there should be a mushroom head button, with N.O. contacts, that is used to apply power the Open coil. There should then be a reset button, again with N.O. contacts, that is used to power the Close coil. As soon as either button is released the power to its respective coil is removed.

I have no reason for why an H-O-A switch would be involved.

 

[pjholguin]

I have been thinking about the HOA device mentioned, and the only reason would be to have a set of N.O. contact controlled by a control system...Energy Management System. The contacts would need to be pulsed. JMHO

In my experience, very few of these installations are actually E-stops. Many of the 'Off' buttons are located in restricted area (e.g. shop office), or there is only one for the entire room. Most of these contactors are more in line with the power off circuits we see in data centers.

The primary purpose appears to be to turn off the shop equipment so it cannot be used outside of allowable hours. Yes, emergency stopping is a side benefit.

For a mechanically held contactor, there should be a mushroom head button, with N.O. contacts, that is used to apply power the Open coil. There should then be a reset button, again with N.O. contacts, that is used to power the Close coil. As soon as either button is released the power to its respective coil is removed.

I have no reason for why an H-O-A switch would be involved.

 

[texie]

It seems to me that a mechanically held contactor is a poor choice for this application. If the power fails the system will restart on its own which can be a safety issue. I think a better choice would be an electrically held contactor with 3 wire control. That way even if you have a power "blink" if can't restart unexpectedly.

 

[fmtjfw]

These E-Stops are common in vocational classrooms -- machine shops, electrical course ....
Often they are implemented using shunt trip breakers that feed the panelboard(s) for all non-lighting circuits in the classroom.

 

[Jraef]

I've seen a similar thing at my son's middle school wood shop when I was asked by the teacher to help trouble shoot it. Each individual machine has its own specific safety/E-stop system, this switch was basically a backup "enable /disable" system as Jim Dungar explained. The teacher, when leaving the room at the end of the last class of the day, pushes the button and unlatches the contactor, rendering the 208 or 240V power tool outlets dead, but leaving the regular 120V outlets hot for the janitorial staff. It so happens to ALSO work in an emergency, but is not the MAIN safety system.

The HOA switch is there mainly for testing. In Auto, this "E-Stop" (or really "All Stop") switch is in the circuit with a set of Fire/Emergency Alarm system control contacts. So if someone pulls a fire alarm switch anywhere on campus, all the big power tools are killed, because even if the emergency (like, unfortunately a shooter on campus) is somewhere else, the tool noise might make it so that the people in the shop can't hear announcements. But the Alarm system often has broken wire detection and you don't want to have to trigger the alarm system every time you want to test this system, so once a month you put it in Hand, slap the button, see that it works, then put it back in Auto.

As to it being mechanically latched and thus requiring power to unlatch (open), it's likely because it is basically On (closed) all the time, which can burn out coils on electrically held contactors, especially those with multiple (more than 4) poles that need stronger magnets to pull them in. That's why many multi pole lighting contactors are mechanically held too. Since it is NOT the primary safety/E-Stop system, that's fine.

So as to how to wire it, you would wire the NO contact of this E-Stop button to the "Un-Latch" coil terminal in parallel with the Alarm system contact, and place the NC contact of the switch in series with the "Latch" coil. But only if I am right about all the rest of this. You really should have it all investigated by whomever designed it all, or is charged with the safety of the shop. If you do your own interpretation and you are wrong, you make yourself liable.

 

[Besoeker]

As to it being mechanically latched and thus requiring power to unlatch (open), it's likely because it is basically On (closed) all the time, which can burn out coils on electrically held contactors, especially those with multiple (more than 4) poles that need stronger magnets to pull them in.

Two points.
If the contactor is to disconnect the whole supply to the lab or whatever, I can't see why need do more than interrupt three phases.
We have such a system in various parts of our works. In the event of a problem, someone/anyone can whack that red button and a contactor opens all three phases or single phase depending on the supply.
The button is latching and has a key release. That needs an authorised person. I have appointed just three. Myself not being one of them for obvious reasons.

The coil burning out...........
Our drives are mostly used in continuous process industries....24/7.
They have output contactors, a safety requirement in many specifications, to allow a physical disconnect between the drive panel output terminals and the motor.
I don't recall a single instance of a continuously energised output contactor coil burning out. I'm not for a moment suggesting that it never happens. But, based on my experience, I think it is a pretty infrequent occurrence.

So, I'm still at the same place.
Why use a mechanically latched contactor when a conventional contactor would be, on the face of it, a better fail safe option?

 

[pjholguin]

The mechanically held contactor does not have power to coil all the time. It is held in place by a magnet which requires a pulse to latch and
a pulse to release. If you are stating the condition of the of the contactor, yes it is in the closed position but requires a pulse to release. As for
this case it sounds like the mechanically latched contactor is not powering any coils to be held closed. JMHO

[/QUOTE] As to it being mechanically latched and thus requiring power to unlatch (open), it's likely because it is basically On (closed) all the time, which can burn out coils on electrically held contactors, especially those with multiple (more than 4) poles that need stronger magnets to pull them in. That's why many multi pole lighting contactors are mechanically held too. Since it is NOT the primary safety/E-Stop system, that's fine.

So as to how to wire it, you would wire the NO contact of this E-Stop button to the "Un-Latch" coil terminal in parallel with the Alarm system contact, and place the NC contact of the switch in series with the "Latch" coil. But only if I am right about all the rest of this. You really should have it all investigated by whomever designed it all, or is charged with the safety of the shop. If you do your own interpretation and you are wrong, you make yourself liable.[/QUOTE]

 

[jim dungar]

...better fail safe option?

It is not a safety device.
It is a master enable relay, maybe. There is no life safety intended to be directly involved in it operation or lack thereof.

Its purpose to do turn off a large area of power at one time just like a lighting circuit.

Automatic restart of the power tools is handled, per OSHA, at each power tool cord.
These have been in use for school shops for maybe 60 years or more.
The function could be achieved with a shunt trip breaker installed (as mentioned by Jraef), other than the hassle of having to turn a breaker on and off versus simply pushing a button.

 

[Besoeker]

Its purpose to do turn off a large area of power at one time just like a lighting circuit.

OK.
I'm still at a loss as to why a mechanically latched contactor would be the better option.

 

[Caleb907]

Attached pics of contactor

Attached pics of contactor

This is what was engineered for the wood, metal shops, and science room. These contactors are pre wired and labeled accordingly, and sent with these momentary mushroom head e-stops. I just don't understand how it works with just that and not the integration of another system or component.

 

[Jraef]

OK.
I'm still at a loss as to why a mechanically latched contactor would be the better option.

You are comparing this to a contactor used in a drive, so maybe up to 4 poles, which would not be a problem being energized all the time. But what is done with these contactors is not to kill ALL of the 3 phase power, it kills ONLY the circuits feeding the bench power tool outlets, and there may be 10 of them in the classroom, so 20 poles. Rather than killing ALL power, they put in a 20 pole contactors (or 2 x 10 pole) connected only to the circuits for those outlets, and as I said, leave the standard 120V outlets alive for the janitorial crew. If you have a 20 pole contactor or even a 10 pole contactor, the coil force required to pull it in leaves you with a high coil current draw for holding too, and that means heat. A mechanically latched contactor has zero holding current, no heat.

 

[jim dungar]

OK.
I'm still at a loss as to why a mechanically latched contactor would be the better option.

Maybe you need to define why it is worse?

Its advantages include:
If the device is never operated then why waste the electricity to run the coil (honestly this is was part of a sales pitch for mechanically held contactors).
If the device is located in the panelboard in the classroom, a mechanically held device would not have the 'hum' associated with that of a large (+100A) device's energized coil (another typical selling point).
After a power outage, the device does not need to be reset. Anecdotally, I know of installations where these devices have never been turned off (I was in one school, where the shop instructor did not know where the reset button was located).

 

[jim dungar]

This is what was engineered for the wood, metal shops, and science room. These contactors are pre wired and labeled accordingly, and sent with these momentary mushroom head e-stops. I just don't understand how it works with just that and not the integration of another system or component.

Your contactor has been supplied with an auxilliary relay to create a 2-wire control circuit rather than a standard mechanically latched-unlatched one.

This is not the correct scheme to use with a momentary pushbutton.

If your button was a latching type, then you would simply wire it (using Normally Closed contacts) into the circuit where the '2-wire control device' is shown.