Combining AC / DC signals in single conduit.
Combining AC / DC signals in single conduit.
(OP)
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?
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?





RE: Combining AC / DC signals in single conduit.
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.
RE: Combining AC / DC signals in single conduit.
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.
RE: Combining AC / DC signals in single conduit.
This may help...
RE: Combining AC / DC signals in single conduit.
RE: Combining AC / DC signals in single conduit.
RE: Combining AC / DC signals in single conduit.
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.
RE: Combining AC / DC signals in single conduit.
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
My WEB: <http://geocities.com/nbucska/>
RE: Combining AC / DC signals in single conduit.
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.
RE: Combining AC / DC signals in single conduit.
what is the input impedance?
Threshold? Is there input hysteresis?
Acceptable delay?
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Combining AC / DC signals in single conduit.
RE: Combining AC / DC signals in single conduit.
RE: Combining AC / DC signals in single conduit.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Combining AC / DC signals in single conduit.
RE: Combining AC / DC signals in single conduit.
and use the longest delay you are comfortable with.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Combining AC / DC signals in single conduit.
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.