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ESD Button Grounding

ESD Button Grounding

ESD Button Grounding

(OP)
We have a mulitcore cable coming from an interposing ESD relay going to multiple pushbuttons in field. The ESD relay is having a 120VAC input and the pushbutton contacts in the field are powered by the relay internally at 24Vdc via rectifier internal to the relay. The pushbuttons in the field are wired in series so if one button is pushed, it breaks the circuit and de-energize the ESD interposing relay. Two cores from the control cable is used. A third core is used for grounding and act as a return path. Separate cable were used to interconnect the pushbuttons in series and so this third core is also interconnected. However, in the last pushbutton, a 4th core is used from the home run cable and this creates a ground loop return to the relay. Is there any issues with this since this grounded cores are just used for protective earth and not as a shield ground? The instrument engineer is concerned that it may create step voltages when one of the current carrying cores is accidentally grounded.

RE: ESD Button Grounding

He is a newbie, I assume? Such things are much exaggerated by some guys.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: ESD Button Grounding

(OP)
I just talked to the concerned person and it took me up to the "instructors" level before he finally got convinced that there is nothing to worry about.

RE: ESD Button Grounding

What was his reason to be concerned? The infamous 'ground loop' thinking?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: ESD Button Grounding

(OP)
That's exactly what he was thinking which I have to correct him "Electrically".

RE: ESD Button Grounding

I am having trouble visualizing the circuit. I am not even sure how many cables are involved.
It is not a good idea to use a ground conductor to complete a circuit. However a circuit conductor may be grounded.
For the push buttons in series, there should be two wires. One of them may be grounded, but a grounding conductor should not be used for one of these wires.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: ESD Button Grounding

PS. My concern is possible failure modes and good practice. I agree with Gunnar that step voltages are probably not an issue.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: ESD Button Grounding

Thank you for the clarification.
I see a code issue here. North American codes require all circuit conductors to be contained in the same cable.
But those codes may not apply here.
There may be a voltage induced in the push button conductor in the event of a short to ground but I agree with Gunnar that it will probably be negligible. It has been a couple of generations since static logic circuits reacted to such small levels of step voltages and then it was often more of an issue if the power supply produced a step.
The GE CR245 line of static logic modules comes to mind as an early design that was particularly susceptible to step voltages. This stuff could reset it's bi-stable gates with a step of a few volts on the power rails. Those days are long gone.
Again, I agree with Gunnar that step voltages are not an issue.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

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