Using the shield as a ground conductor?

[slongo]

I have come across several electrical schematics that use the shield of a twisted pair (16 AWG instrumentation wire) to ground a 120 VAC proximity switch (as an example) back to the plc. There are 2 main types of shields: braided (an actual copper conductor) or the foil with drain wire. I would assume that if the shield is a braided conductor sized to carry the current in the circuit, then it's OK to do? Is this acceptable practice? Please help. I never found clarification in the NEC.

 

[dereckbc]

Re: Using the shield as a ground conductor?

The shield is not to carry current. Most design applications only require the shield to be bonded at one end only to prevent circulating ground current or noise.

 

[hurk27]

Re: Using the shield as a ground conductor?

Dereck

In certin class 1,2 or 3 circuits it is allowed as it will only carry the fault current and not the load. as per 725

 

[dereckbc]

Re: Using the shield as a ground conductor?

Wayne as I said "design applications". Yes the code does allow it.

 

[Len_B]

Re: Using the shield as a ground conductor?

For a discrete(on/off) inputs or outputs operating at 24 volts or greater(including your prox switch) this shouldn't be a problem, though I wouldn't consider it good design practice as Derick already stated.
For all analog I/O (and possibly 5 volt discrete) this could create big problems(induced voltages/currents) and should definitely be avoided.

Len

 

[tom baker]

Re: Using the shield as a ground conductor?

Thats not the function of the shield. Why not use a 3 conductor shielded cable with the 3 conductor as an equipment ground? Of if its in raceway pull an insulated equipment ground.

 

[renosteinke]

Re: Using the shield as a ground conductor?

The shield is not intended to be a grounded conductor- using it as such defeats its' purpose.
The shield is, in effect, an antenna that will prevent nearby transients from inducing a false signal on the line. Ground it at both ends, and it's not an antenna anymore!
PLC's, in any event, work with micro-voltages and no currents. Where is the need for grounding?

 

[tom baker]

Re: Using the shield as a ground conductor?

Reno, grounding will be required if the circuit from the PLC operates at above 50 VDC or in a hazardous location.