Emergency switch

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Thread Starter

Tanweer Ahmed

Hello list members,

At our plant we are facing problem with our hard wired emergency stop circuit. Due to vibration the NC contact opens for a fraction of second
and the emergency is activated without any indication to the PLC system. I will be thankful if any of you can provide me with make and model of contact blocks or switches suitable for this application. Is there any standard specification of NC emergency switches that can sustain enough jerks and vibration.

Tanweer Ahmed
Phone: +9221-636 5519
Fax: +9221-568 2972
E-mail: [email protected]
 
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Lajos Gyarmati

My favorite E-Stop switch is the one from GE
C-2000 Series Push Buttons. All the details are in the GE Industrial Systems Control Catalog.
Catalog #: GEP-1260L

Quote from the specs:
"Resistance to vibration: Per IEC 68-2-6.
16g with a frequency from 40-500 Hz and
maximum peak to peak amplitude of 0.75mm.
Resistance to shock:
According to MIL 202B, method 202A.
Test was performed for 1/2 sinusoid for 11ms,
38g max for all operators with transformers
and 100g for all other operators.

About the contacts:
Ultrasonically welded contact block housings hold up longer and resist contamination better, even in the dustiest enviroments. Double bridge sliding contacts provide excellent perfomance, even in low-voltage applications."


I have used these switches many times on different types of machine tools and they are very reliable.

I have experienced however problems with different brands when the E-stop buttons or any normally closed pushbuttons were used in 24V or lower DC circuits on machine tools. Humidity, dirty air and time can oxidize the contact surface and create contact problems in these low level circuits. The very same switches function much longer without failure when the control voltage is 115V AC. For this very reason I prefer to build the Estop circuit on 115V AC level for all of our machines.

Lajos Gyarmati
[email protected]
 
J
Are you using the hard wired E-stop circuitto a PLC input, or is it dropping the controlpower to the output cards?
If you are monitoring a PLC input for a change of state you could put a timer on it to buffer out the noise. You would only need to set the timer for .25 seconds to filter out the gilches while still having acceptable response to a true e-stop condition.

Jeff Jones
Control Systems Tech.
 
A

Anthony Kerstens

There are several things to try:

1. thick rubber gasket on PB mounting.
2. mechanically remounting the box, with gaskets
or isolated from vibration.
3. try a safety timing relay. Honeywell makes such
a beast.

Anthony Kerstens P.Eng.
 
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Jose A. Castellan R.

Hi,
I suppose that your NC contact is electromechanical, if this is the situation you could replace it for a solid state NC contact and forget any type of vibration.
Regards

Jos=E9 A. Castell=F3n R.
TecSim
[email protected]
 
T
Tanweer,

Modern Emergency Stop Switches, (e.g. CE Marked), all have "Anti Teasing" contacts, i.e. they must be positively pressed in order to break the normally closed contact. It is unlikely that these would fail from vibration other than
extremely high level impacts. Please check the quality of the switch that you are using, the condition of its contacts and all of the wiring for possible intermittents. If switch has 2 poles, or can have 2nd contact set added, then
wire both sets in parallel for some redundancy. Also, it should be possible to shock mount the switch by mounting the switch plate on e.g. rubber bushings.

Tony Firth, Electrical Engineer,
Quester Technology Inc.,Fremont,CA
 
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Tanweer Ahmed

Hi Tony,
My Switches are CE marked and they are Siemens manufactured. We have gone through extensive testing of the whole circuit. These contact blocks really make problem. However your idea to put two contact blocks in parallel is convincing.

Thanks,

Tanweer Ahmed
Phone: +9221-636 5519
Fax: +9221-568 2972
E-mail: [email protected]
 
M

Marc Sinclair

> intermittents. If switch has 2 poles, or can have 2nd contact set added, then
> wire both sets in parallel for some redundancy

I expect you meant series, putting two nc contacts in parallel would degrade safety. I think you were right about the quality aspect, we've always used the klockner RMQ 22mm range and never had a problem.

.marc sinclair
 
Tanweer,

We use two Emergency switches in the console of the operator desk with 2 sets of contatcts in furnace protection. one set directly wired to trip circuit. In such cases each of the set has changeover contacts which can be wired depending on the process requirement. like for NC it is wired in poarallel and in case of NO it is wired in series. The make what we used is M/S H&B make console mounted emergency stop button.

This has both redundancy and availability built in due to 2 switches and wired in series or parallel.

thanks in advance

[email protected]
 
M
Every switch manufacturer probably has a 'late break' contact block suitable for ESTOP contacts. For the AB800T series it is an XD4 block. We have used the 800T-FX6D4 for about 18 years with good success.

(We did have problems with Furnas switches, but
I think they had the 'normal' NC contact block.)

Mark
 
hello:
Another simple means we follow is to wire two such switch contacts in series. The operator has to press both these switches simultaneously in the event of Emergency. Normally we use Mushroom head Push Buttons. This also to some extent prevent nuisance tripping due to vibration.
Regards

Santhiraj
 
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Anthony Kerstens

You can't guarantee how a solid state relay is going to fail. The application is for a safety circuit, is it not?

Anthony Kerstens P.Eng.
 
You might try Rees pushbuttons. They are very simple and rugged. I'm not sure if it will solve your problem, but they are hard to beat for ruggedness. I see them used frequently for e-stop buttons.

Bill Sturm
 
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Tanweer Ahmed

Hi Anthony Kerstens,

The idea of shock absorbers are good but I don't want to put any delay timer with E-stop circuit because emergency is emergency and every millisecond to that is important.

Thank you,

Tanweer Ahmed
Phone: +9221-636 5519
Fax: +9221-568 2972
E-mail: [email protected]
 
A

Anthony Kerstens

If they are in series, then wouldn't only one PB need to be pressed?

Also, in an emergency situation, is an operator going to have the presence of mind to press both buttons? What about a visitor or new employee?

Such an arrangement is inherently unsafe.

Anthony Kerstens P.Eng.
 
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Tanweer Ahmed

Hello Nelson,

We don't have individual machines but its an integrated process plant and almost all the emergency push buttons are mounted on control desks.

Tanweer Ahmed
Phone: +9221-636 5519
Fax: +9221-568 2972
E-mail: [email protected]
 
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Tanweer Ahmed

Hello Marc Sinclair,

By series I mean all the E-stop push buttons 22 in number located plant wide are connected in series.

Tanweer Ahmed
Phone: +9221-636 5519
Fax: +9221-568 2972
E-mail: [email protected]
 
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