Intrinsically safe circuit - surge protector

nhee2

Senior Member
Location
NH
A field installation has an intrinsically safe RS-485 communications circuit to a field mounted device. The proper IS barrier meeting the requirements of the control drawing is installed at the panel (non-hazardous) end of the circuit. At the field end (I.S.), a passive (non-powered) data line surge suppressor is installed.

The control drawing for the field device doesn't include any provisions for the surge suppressor - even though it is passive, does it need to be explicitly identified on the control drawing for it to be consistent with the approved configuration?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
A field installation has an intrinsically safe RS-485 communications circuit to a field mounted device. The proper IS barrier meeting the requirements of the control drawing is installed at the panel (non-hazardous) end of the circuit. At the field end (I.S.), a passive (non-powered) data line surge suppressor is installed.

The control drawing for the field device doesn't include any provisions for the surge suppressor - even though it is passive, does it need to be explicitly identified on the control drawing for it to be consistent with the approved configuration?
I would say yes.

PS you can't just move it to the other end of the wires either.
 

nhee2

Senior Member
Location
NH
yes, I agree, that is how I was leaning. This seems like one of those scenarios where the surge suppressor doesn't negatively change the circuit, but since it is not listed it can't go there. although I wonder if the MOVs or zeners or whatever is in the surge device would affect time-response of the barrier.
 

paulengr

Senior Member
yes, I agree, that is how I was leaning. This seems like one of those scenarios where the surge suppressor doesn't negatively change the circuit, but since it is not listed it can't go there. although I wonder if the MOVs or zeners or whatever is in the surge device would affect time-response of the barrier.
MOVs are ineffective at low voltage. It will be a PIN or fast recovery diode or a GDT or spark gap. Of those the diodes have some capacitance so technically a violation but it’s so small I’d ignore it. The spark gap is obviously a bad idea. That leaves the good old telco GDTs (gas discharge tube) which is essentially a hermetically sealed spark gap and what you probably should use since it won’t violate IS since there is no stored energy.
 
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