Using the shield as a ground conductor?

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slongo

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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

Moderator
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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

Senior Member
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

Moderator
Staff member
Re: Using the shield as a ground conductor?

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

Len_B

Member
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

First Chief Moderator
Staff member
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

Senior Member
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

First Chief Moderator
Staff member
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.
 
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