MULTIPLE EMERGENCY OFF SWITCHES
MULTIPLE EMERGENCY OFF SWITCHES
(OP)
I have 30 switches in series on a long belt conveyor that runs more than 500 feet. The total length of the circuit is 556 feet, 2 wire, 24vdc, 5 amp supply. I am using 18AWG wires and any of these 30 switches can be activated to turn off the motor.
Since this is a critical function circuit with zero tolerance for failure, do you see any chance of malfunction? Will there be issues such as voltage drop that will prevent proper switch function ? Will DC be OK or AC? And why ?
thanks for your help
murli
Since this is a critical function circuit with zero tolerance for failure, do you see any chance of malfunction? Will there be issues such as voltage drop that will prevent proper switch function ? Will DC be OK or AC? And why ?
thanks for your help
murli






RE: MULTIPLE EMERGENCY OFF SWITCHES
Yes there is a difference on long loops between DC and AC.
Check these two threads as they have a wealth of info.
thread237-102363
thread237-83120
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: MULTIPLE EMERGENCY OFF SWITCHES
With 30 switches, I think it would be most likely, and I would not like to be the maintenance staff.
Is there a reason you are not using a pull-wire system?
RE: MULTIPLE EMERGENCY OFF SWITCHES
Even if your actual current is less than 5 amps, you get the point. In practice, the circuit resistance will limit the voltage drop to the applied voltage, but when the quick calculation shows a voltage drop of almost 350% the circuit probably won't work.
In agreement with previous posts, all devices must be in series for failsafe operation.
120 volts AC is(was?) the standard voltage for industrial controls. It gives a good compromise between safety and reliability.
Many conveyor systems are protected with a pull-wire or a "jerk-line" down either side. The line itself provides a measure of physical protection. Should someone step backwards towards the conveyor, they will feel the line across their back before they exert enough force on it to shut down the conveyor.
Two pull-wire switches will probably be much cheaper safer and more dependable than 30 individual switches.
The pull-wire switches may be arranged at the control end of the conveyor to keep the circuit lengths short.
respectfully
RE: MULTIPLE EMERGENCY OFF SWITCHES
thanks for your help!
RE: MULTIPLE EMERGENCY OFF SWITCHES
Relay coils pickup is usually 80% of rated voltage, dropout (what you would be concerned about) is usually 70%. I show the resistance of 18ga stranded wire as being 6.5227 ohms per 1000 ft., so if the total run is 556', that means only 3.63 ohms. So starting with a 24VDC supply and assuming your 1A load through a resistance of 3.63 ohms you will get a 3.63V drop which is 15% or the coil rating, shouldn't be a problem for pickup or dropout.
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: MULTIPLE EMERGENCY OFF SWITCHES
With a 500 foot conveyor and a 556 foot circuit length, I am guessing that the total wire length is 1112 feet.
I recommend at least 12 awg. wire to reduce the voltage drop.
With 1112 feet of wire, the voltage drop will be 7.26 volts.
I am somewhat skeptical of designing a circuit with 15% voltage drop. (Or 30%)
I recommend using #12 wire to reduce the voltage drop by a factor of 4.
I would prefer to see 120 volts with #14 wire. This is a control arrangement that has given generations of dependable service.
respectfully
RE: MULTIPLE EMERGENCY OFF SWITCHES
respectfully
RE: MULTIPLE EMERGENCY OFF SWITCHES
All I can say is;
D'oh!
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: MULTIPLE EMERGENCY OFF SWITCHES
Hehe
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: MULTIPLE EMERGENCY OFF SWITCHES
RE: MULTIPLE EMERGENCY OFF SWITCHES
Don
RE: MULTIPLE EMERGENCY OFF SWITCHES
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: MULTIPLE EMERGENCY OFF SWITCHES
The relays should be energized at all times and you should be using normaly open contacts on the relays to control your motor starters. Then if anything goes wrong, (Broken wire, failed power supply, or someone pushed the stop button) the relay will be de-energized and open the circuit. If the drive motor is at the far end of the conveyor I would be more concerned with induction than capacitance. Run your controls on the opposite side of the conveyor to the power feed. If possible use the frame of the conveyor to shield the cables.
Another advantage of pull-wires is that they are immune to induction and capacitance is not an issue.
respectfully
RE: MULTIPLE EMERGENCY OFF SWITCHES
There are other factors here besides voltage drop capacitance etc.
Trip wire switches are used for good reasons, you can reach them from anywhere on the conveyor. The first person who get caught up in your conveyor and can't reach the neares E-switch is going to be after you with his lawyer ( or his family will be after you with his lawyer). You will have to explain why he had to go up to 12 ft. to get to a switch, while on every other conveyor in the country you just have to fall on the cale.
There are times when it's best to follow convention and not be creative.
RE: MULTIPLE EMERGENCY OFF SWITCHES
Sorry BJC, couldn't resist the opportunity for a joke even if it was at your enpense. I think you meant MSHA, Mine Safety and Health Admin.
Actually, the e-stop rules vary by conveyor design. Catwalk / no catwalk; cover / no cover; short / long etc. But you are correct, MSHA is the governing body if it is a mine or quarry, ON TOP OF NEC requirements, which still must be met from a purely electrical standpoint.
Also assuming of course that the OP in in the US...
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: MULTIPLE EMERGENCY OFF SWITCHES
http://eca
Regards
Ralph
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
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RE: MULTIPLE EMERGENCY OFF SWITCHES
RE: MULTIPLE EMERGENCY OFF SWITCHES
thanks