MULTIPLE EMERGENCY OFF SWITCHES

[smur]

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

 

[itsmoked]

For safety reasons I hope these switches all have to be closed for the conveyor to run. That way just having the conveyor operate helps demonstrate the circuits integrity.

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

 

[DaveScott]

//do you see any chance of malfunction?//
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?

 

[waross]

Well, a quick extrapolation from my voltage drop tables, using 5 amps and #18 wire shows a voltage drop of 41.7 volts. For a 500 ft. conveyor I suspect that the total wire length will be 1112 ft. The voltage drop will be 83.4 volts.
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

 

[smur]

Thanks for all the information. Yes I agree it is better to go for the pull wire system. But due to reasons beyond my control I will have to use this system. All switches are Fail safe. I have 3 relays that are using this circuit and they are small. May be 1 amp at the most. Will it work for 1 amp current ? Or if I go for which of the wires such as 12, 14, 16 will mitigate this issue?
thanks for your help!

 

[jraef]

The amps in the circuit have nothing to do with the power supply other than a (remotely) possible overload issue. The amp draw has only to do with the load on the circuit, i.e. the relay at the end. I cannot for the life of me believe that a relay coil is going to pull 5A!

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

 

[waross]

Hi jraef;
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

 

[waross]

thread237-164753 has a discussion of the merits of 24 volts vs. 120 volts.
respectfully

 

[jraef]

With a 500 foot conveyor and a 556 foot circuit length, I am guessing that the total wire length is 1112 feet.

All I can say is;

D'oh!

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

Dang! I need another spool?? No wonder I got the bid..

Hehe

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[smur]

Thanks for all these great responses. Aprreciate. but is there going to be problems assocaited with cable capacitances? How would a long cable capcacitance affect ? I heard form some of the messages that the contactor or relay fails to operate due to cable capacitance. In my case what is the capacitance and how will this prevent the operation of the relay if at all ? Is this due to the fact that voltage cannot change quickly just like the current cannot cahnge in an inductive circuit quickly enough ? Would a long circuit create a leading power factor situation and thus hard for the voltage to change ? If I use 12AWG I will have less voltage drop, but will realy operate to open emergency circuit? All the time?

 

[resqcapt19]

Square D had a great paper on the capacitance effect on circuts of this type, but my link to it no longer works. If anyone has a current link it would be a great addition to this thread. There is a link to it in one of the previously referenced threads, but it is the same one as I have.
Don

 

[itsmoked]

smur that is why we keep stating 24VDC the capacitance issue goes away in your case.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I am concerned with your concern about the relays operating.
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

 

[BJC]

Is this conveyor in a mine? IF so you have MENSA govering the installation not the NEC or other body.

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.

 

[jraef]

MENSA? The High IQ organization? Man, those brainiacs are getting into everything now!

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

 

[RalphChristie]

resqcapt19:

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680079349

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[resqcapt19]

Thanks Ralph.

 

[smur]

Thanks for all the information. You have talked about AC circuits. How about DC circuit power source ? i am using 24VDC power and not AC. Will this make any difference ?
thanks