Emergency Push Button with Padlock 1

[NickParker]

Where would you apply a Emergency Push button with padlock?
I have never seen Emergency Push button (stay put) with padlock for MV & LV Motor applications. Recently came across a spec mentioning it.

 

[mparenteau]

Lock out / Tag out.

Mike

 

[JezNZ]

As noted above. But only where client safety procedures allow the use of Emergency Push Buttons as an isolation devices - this is a hotly debated topic (in my experience) but generally considered safe where there is emergency circuit monitoring and SIL-rated control system.

 

[stevenal]

Here's what OSHA says about an e-stop input to a PLC and LOTO.Link

 

[BrianPetersen]

I have seen it used.

The above OSHA link doesn't distinguish between a "PLC" and a "safety PLC".

The E-stop button, with appropriate underlying components, circuits, and logic is OK for preventing something within its span of control from "moving" under power if you lock it from being released. If that's the purpose for which the lock-out is being applied (e.g. to service mechanical parts of the system), that's one thing. It's quite another if someone is doing electrical work. It will NOT prevent electricity from being supplied to the panel that contains these components, and possibly not stop electricity being supplied in some form to the motor (just not necessarily in a form that will cause the motor to rotate - see "safe torque off" on a VFD or servo drive, for example).

Local standards and locally accepted practices may dictate otherwise.

If locally applicable standards permit the use of a padlock over an E-stop button then at a minimum the written lock-out procedure (you have one, right?) needs to distinguish between performing mechanical work and performing electrical work.

Ultimately there is no substitute for the big ole' disconnect switch with a padlock on it.

 

[che12345]

Dear Mr. NickParker (Electrical)(OP)18 Jul 22 18:54
"...Where would you apply a Emergency Push button with padlock? I have never seen Emergency Push button (stay put) with padlock for MV & LV Motor applications. Recently came across a spec mentioning it."
1. The intention of an E-stop push-button usually of mushroom head and stay-put design is that any person trained/untrained qualified/unqualify, to struck it when noticed a danger or potential danger to life or machine, threatening fire etc., to switch off the electrical supply and to stop the machine immediately; instead of searching for the breaker or the switch.
2. E-stop has nothing to do with LOTO. E-stop must not be used as a on/off or start/stop push-button.
3. Having say that, I have seen E-stop push-button having a transparent simple easily flip-up ( NOT lockable) cover over it. It is to prevent accidental struck.
4. Ultimately you may have to ask the specification writer what he has in mind and what is the rational/reason behind it. There may be some good reasons or there is a need for it. But pad-locking it from operation in simple term, is a no, no ,no.
Che Kuan Yau (Singapore)

 

[JezNZ]

@che12345

1. agreed.
2. In some sites/parts of the world/industries E-Stop are used as LOTO devices. This doesn't mean they are used for start/stop or on/off operation.
3. agreed.
4. Typically lockable E-stop are lockable ONLY in the Engaged/On/E-Stop position.

As predicted, hotly debated topic.

 

[FreddyNurk]

I've seen both sides of the argument in place, I recall in some mines they'll have conveyor trip wires as well as periodic E-Stops (as an example) and in such industries using the E-Stop as an isolation point is not accepted. My (possibly incorrect) understanding is that the stop mechanisms can sometimes be bypassed until such time as further action can be taken to verify the issue.

Other industries will use the E-Stop as one of the isolation points, usually in addition to others such as fuel supply valve, circuit breaker and so on. In such cases the E-Stop is lockable in the stopped position and thus becomes part of a Lock Out Tag Out system.

In all cases the isolation points must be designated and understood, as well as easily accessible for the task at hand. I tend to agree with BrianPeterson though, as far as removing potential sources of energy, using an emergency stop facility doesn't really meet that function.

EDMS Australia

 

[che12345]

@ JezNZ (Electrical)19 Jul 22 00:31
"....#2. In some sites/parts of the world/industries E-Stop are used as LOTO devices. This doesn't mean they are used for start/stop or on/off operation....#4. Typically lockable E-stop are lockable ONLY in the Engaged/On/E-Stop position..."
Thank you for your learned advice. I have the following opinions.
#2. In all cases I noticed that E-stop with NC/NO contact is inserted in the control circuit. It breaks the supply to the e.g. under-voltage release/contactor or energizes the shunt-release; to open the switching device. It is an emergency action to stop the machine/operation immediately. As for LOTO it is to ensure workers' safety, when working on the machine; a non emergency issue. It should be locked out by mechanical means blocking the switching device handle from closing. In addition, for draw-out breakers also mechanically blocked/locked from drawing into service position. By mechanically locking the switching device operating handle preventing it from closing is far safer than depressing the E-stop and locking it.
#4. Agreed. It make sense, as an additional precaution.
Che Kuan Yau (Singapore)

 

[BrianPetersen]

3. Having say that, I have seen E-stop push-button having a transparent simple easily flip-up ( NOT lockable) cover over it. It is to prevent accidental struck.

Locally (Canada), this is absolutely not permissible. CSA Z432-16 7.15.2(b)

I think IEC 60204-1 and ISO 13850 may have something to say about the matter as well, but I don't have copies of those handy at the moment. I did find this, which covers the issue (as well as addressing the lockable design mentioned in the original post):

https://www.idec-emea.com/img/cms/IDEC/Expertise/EF3001-0_whitepaper_0225.pdf

The question of course has come up: "Can we put a shield around/over this E-stop button, because people keep hitting it when walking past the machine" ... No, but you can orient the button the other way or face it upward instead of outward so that the natural path of someone walking past the machine doesn't involve their hip striking the button ...

 

[LittleInch]

I agree that this is not a great way to lock out a motor, but also that there is a difference between a single direct wire into some sort of relay where once you lose power the relay drops out and cuts off the power vs switches which go into a PLC of any sort.

I had a project where the ESD trips kept going off and it was only then that I realised what I thought was a simple on off switch, was in fact a switch which instead sent the current via a resistor and the difference in voltage in the loop ( we had a few buttons in a row) what what was being detected in the I/O panel and a signal sent to to the PLC. It meant you could actually identify which button had been pressed as each button had a different resistance, but also meant we could override the buttons via the PLC....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BrianPetersen]

Any PLC involved in an emergency-stop function, had better be a safety PLC with a declaration of conformity to the machinery directive that references ISO 13849-1 and declares a PL appropriate to the risk assessment! And the logic needs to be right, too.

 

[jraef]

I have seen lockable E-Stop buttons on large motor applications used for MECHANICAL work LO/TO procedures because the only disconnect is a cell in draw-out switchgear and racking out the cell to do a full ELECTRICAL isolation can only be done by a qualified electrician wearing proper PPE etc. etc. etc.. OSHA allows the lockable E-Stop in this kind of situation (I know because I challenged it and was shown the ropes). As I recall the wording (it was 20+ years ago now), it was something to the effect of "where requiring electrical isolation for mechanical work is not practical, a locking Stop button can be used..." (grossly paraphrased). Mine was at a pump station with 800HP 480V pumps. Draw-out switchgear fed Soft Starters, which fed the pumps. The only disconnect means was the draw-out cell in the switchgear and only one of 3 electricians was qualified to draw out a cell. So next to each pump there was a push button station with an E-Stop button and a padlock hasp for when they were doing mechanical work on the pump. Any electrical work required drawing out the cell, but of course that would only be done by the electricians, who were allowed to do so.


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