proximity switches wiring

[lpena]

I have some discrete valves that need to have proximity switches to indicate the valve's position (open or closed). My customer is requiring me to use shielded cables to wire the proximity switches to the PLC. Is that required for proximity switches? We have used control cables (unshielded) in the past for limit switches. The signal type for the proximity switches is 24VDC. Thanks

 

[GoldDigger]

I have some discrete valves that need to have proximity switches to indicate the valve's position (open or closed). My customer is requiring me to use shielded cables to wire the proximity switches to the PLC. Is that required for proximity switches? We have used control cables (unshielded) in the past for limit switches. The signal type for the proximity switches is 24VDC. Thanks

Limit switches (at least the mechanical contact type) have a very low impedance when closed and are not particularly vulnerable to transients and noise on the signal lines.
Proximity switches with a solid state mechanism will have a higher output impedance (and therefore produce a signal more vulnerable to noise) and the electronics could be damaged by induced voltages on the sensor lines (if the wired went through an electrically noisy area).
The combination of those two factors may be the reason that coax (or at a minimum twisted pair) wiring is recommended.
The type of input to the PLC could also affect this, as you recognize.

 

[Jraef]

Limit switches (at least the mechanical contact type) have a very low impedance when closed and are not particularly vulnerable to transients and noise on the signal lines.
Proximity switches with a solid state mechanism will have a higher output impedance (and therefore produce a signal more vulnerable to noise) and the electronics could be damaged by induced voltages on the sensor lines (if the wired went through an electrically noisy area).
The combination of those two factors may be the reason that coax (or at a minimum twisted pair) wiring is recommended.
The type of input to the PLC could also affect this, as you recognize.

Corollary:
Using shielded cable when you may not have needed it causes no problems, it just costs a few pennies more up front.
Using unshielded cable when you needed shielded can cost you hundreds of times more in troubleshooting and down time than the difference in cable was worth.

 

[Smart $]

Corollary:
Using shielded cable when you may not have needed it causes no problems, it just costs a few pennies more up front.
Using unshielded cable when you needed shielded can cost you hundreds of times more in troubleshooting and down time than the difference in cable was worth.

:thumbsup::thumbsup::thumbsup:

 

[Tony S]

High frequencies or distorted power frequencies from early VSD’s have given me various PLC problems in the past.

As per usual, “more information required.”

 

[petersonra]

I have some discrete valves that need to have proximity switches to indicate the valve's position (open or closed). My customer is requiring me to use shielded cables to wire the proximity switches to the PLC. Is that required for proximity switches? We have used control cables (unshielded) in the past for limit switches. The signal type for the proximity switches is 24VDC. Thanks

There is no code requirement to ever use shielded cable, short of having 110.3(B) invoked.

IME, common prox switches are not especially susceptible to noise anyway.

However, valves sometimes come with switches on them that are not of the common prox variety and it could well be advisable to run them in shielded cable. I would take a look at the instructions and see what they say.

 

[iwire]

I would just do as the customer requests and charge accordingly.

 

[petersonra]

I would just do as the customer requests and charge accordingly.

Generally you can buy common premade cables for prox switches that just plug into the switch body so it is very easy to hook them up. I have never seen one of that style of cable that comes in a shielded variety, although likely you can get them.

The premade prox switch cables are dirt cheap. It occurs to me though that they are probably not a NEC recognized wiring method.

 

[gar]

150929-1517 EDT

If the prox switch has an RTL (Resistor Transistor Logic) type of output, then it is much more sensitive to noise in the high state than if it has a TTL output (Tranaistor Transitor Logic (Totem Pole) ).

Twisted pair wiring reduces magnetic coupling, but not electrostatic coupling (a shield does that). A shield does very little magnetic shielding at low frequencies.

A coaxial cable does pretty good shielding (attenuation) of both magnetic and electrostatic fields. Really coax does not shield a magnetic field, but cancels the induced signals which is what a twisted pair does, but just different ways of canceling for each.

RC filtering at the destination or other signal processing can eliminate some signal noise problems.

Many prox switches are 24 VDC units and because of the high signal level are not too sensitive to noise. The 120 V units are even less sensitive. Other problems are from ground path noise. Don't ground any signal lines at the prox switch. The shell of prox switches is not connected to the internal electronics.

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[petersonra]

150929-1517 EDT

If the prox switch has an RTL (Resistor Transistor Logic) type of output, then it is much more sensitive to noise in the high state than if it has a TTL output (Tranaistor Transitor Logic (Totem Pole) ).

Not many RTL or TTL prox switches out there these days. TTL is 5 V logic and fairly susceptible to noise IME.

 

[gar]

150939-0814 EDT

petersonra:

You did not understand what I said. RTL and TTL are not specific to a particular voltage level, rather they are an output circuit concepts.

RTL has a pull-up resistor and a pull-down transistor. In the high state the output impedance is the value of the resistor, and in the low state it is the parallel combination of the pull-up resistor and the low impedance of the on state of the transistor.

A TTL output circuit has both a pull-down transistor and a pull-up transistor so the output impedance is low in both states. Usually not quite as low in pull-up as pull-down.

Both types of circuitry were generally built as integrated circuits at the 5 V level. But the same concepts apply at 24 V or 120 V or whatever voltage level is practical to work at.

TTL does provide better noise immunity than RTL.

The difference between RTL and TTL is the difference between a SPST switch applying a voltage to a load (low impedance when the switch is closed, that of the voltage source, and high impedance of the open circuit, largely the wiring capacitance), and a SPDT switch that connects the load to the high end of a voltage source (low impedance) and the low end of the source (again a low impedance).

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