Combining AC / DC signals in single conduit.

[eeJason]

Hello,

I would like to combine a circuit driving a solenoid valve at 120VAC in the same conduit with a limit switch signal operating at 24VDC. All the wires are 300V rated.

The customer is concerned that the wires for the AC solenoid valve will induce a voltage in the limit switch wires, possibly enough to cause false signals at the PLC. He would like for me to route the circuit in seperate conduit.

I agree that seperating the circuits would be best, but I also can't imagine that any induced voltage would be significant enough to trigger the PLC input for the limit switch.

Can anyone point me to a standard which would prohibit me from routing these circuits together? Has anyone had a bad experience routing AC and DC signals in the same conduit? What should I do?

 

[dpc]

There are no Code issues, because it is not an unsafe practice.

In most cases this can be done with little concern about induced voltage. However.... your solenoid valve represents about the worst possible load due to the very high inductance. If this is a large SV, there could be problems.

You should do a search in this forum - this was discussed not too long ago and some good suggestions were made regarding preventive measures.

It also depends to some extent on the noise immunity of your dc input.

 

[eeJason]

Thanks for the reply. The coil on the solenoid is on the order of 20W. I wouldn't think this would be considered large.

I looked around, but wasn't able to find any such discussion. If you could refer me to the thread you have in mind it would be much appreciated.

 

[dpc]

thread248-106824

This may help...

 

[cbarn24050]

Its not normaly a problem but if it proves to be you can use the the 24v to drive a relay and wire the relay contacts to the plc.

 

[JLSeagull]

Routing 120 VAC and 24 VDC in the same conduit is a bad practice but should work with less than 1/2 Amp solenoid and position switch contacts. Consider using 600 V insulation. It is difficult to support the use of 300 V insulation except for NEC article 725 power limited or intrinsically safe circuits.

 

[Comcokid]

If you are concerned with the solenoid pulse coupling into into the limit line, you can always change the impedance of the limit input.

In other words, if the limit switch switches to +24 volts when a LIMIT occurs, then put a pull-down resistor at the PLC. Example, a 1500 ohm 2 watt resistor, or even a small lamp which you can label "limit".

The coupling effect between parallel wires will have a fairly high impedance. By adding pull-up or pull-down resistors, you make the input a lower impedance. Similar effect is accomplished by adding capacitors to the I/O except a cap may need to be sized for the pulse width you are trying to supress.

 

[nbucska]

If I understand the geometry the solenoid and a limit
switch are on the same end of the conduit. The switch
has two independent wires (i.e. neither ground nor
a solenoid lead is used to save one wire ).

The lenght of the conduit?

The switch most likely grounds a pull-up R connected
to the imput.What is the resistor value and the switching
threshold?

How much delay could you afford?

Best general purpose solution would be twist the switch
leads ( something like 2 twists/foot) and use a
differential to single-ended comparator with large
hysteresis and a low pass filter on the receiving end.

Depending on the answers to the questions this may
be an overkill.






Plesae read FAQ240-1032
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http://geocities.com/nbucska/>

 

[eeJason]

Unfortunately I don't have the latitude to change the design at this point in any manner other than to adjust conduit runs & conductor/cable types. My question is really whether it would be a dumb idea on my part to argue for combining the circuits in the same conduit to save a run.

JLSeagull, could you explain your comment on insulation? I'm justifying 300V insulation with NEC 300.3(C)(1), which leads me to believe that all conductors need only to have insulation suitable for the highest voltage present (in this case 120VAC).

nbucska, your understanding of the geometry is correct. The equipment is contined on a moveable skid, and the conduit run in question is on the order of 15' in length. The limit switch is a two wire type, connected to an AB 1756-IB32 input card at the PLC.

 

[nbucska]

Is the switch going to GND and is there a pull-up or
what is the input impedance?
Threshold? Is there input hysteresis?
Acceptable delay?


Plesae read FAQ240-1032
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http://geocities.com/nbucska/>

 

[eeJason]

The signal from the switch is dropped across a pulldown resistor. The input impedance is about 5kOhm. The switching threshold is 10V, and the datasheet gives no mention of hysteresis, though it does have a programmable delay that can be used for filtering out noise.

 

[cbarn24050]

You can arrange for your PLC to ignore the switch input when the solenoid is being switched, reduce your resistor to 470 ohm and use the PLC's noise filtering.

 

[nbucska]

Could you please answer finally about the acceptable delay?



Plesae read FAQ240-1032
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http://geocities.com/nbucska/>

 

[eeJason]

That'll be determined largely by the mechanics of the hardware. I can't really give you a hard and fast answer, since the thing hasn't been built yet. I suppose I might be able to tolerate up to 100ms delay or so, though I'd prefer something closer to 10. If you're driving at the filtering solution, I'd prefer to avoid it if possible. If I can expect anything close to 10V worth of cross talk between the two circuits, that's reason enough for me to route them seperately.

 

[nbucska]

Twist the solenoid wires but not the ones of the switch
and use the longest delay you are comfortable with.





Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[JLSeagull]

My first exposure to the 300 vs. 600 volt insulation issue was in about 1983 pertaining to 24 Vdc, 4-20 mA signal cables. This may have been an issue associated with cable tray instead of conduit. Some NEC changes appeared to address this but invariably an inspector somewhere jumps on the issue.

Look at article 725.27 B for Class 1 power limited control circuits where the requirements include 600 V. Later in 725.82 300V is OK for Class 2 and 3 PLTC. However somewhere there are things like the fuse cannot be replaced with one that passes sufficient current to exceed the power limitation. Unfortunately you can almost always find a higher ampacity fuse. Intrinsic safety barriers are among the inspections.

For me it is just easier to go 600 V and forget about all the fine print notes and inspector quirks, etc.