It's a little hard to comment w/o a schematic. But note that under NFPA 79
there are two classes of circuits. It seems like your heater circuit may be
considered a power circuit. Therefore, the inspector could be commenting on
the circuit which controls the "coil" of the 24V solid state relay.
Note also that even though you have two thermal switches on the line side of
the heater coil it's not clear whether they are fail-safe. Redundancy doesn't
add much value if the failure of the switch is not revealed somehow.
Regards,
Lauren Crane
Product Regulatory Analyst
Corporate Product EHS Lead
Applied Materials Inc.
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"Kunde, Brian" <[email protected]>
Sent by: [email protected]
10/27/2008 11:53 AM To
<[email protected]>
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Subject
NFPA 79 Section 9.1.4.1
Dear Experts,
Can anyone please explain to me section 9.1.4.1 of NFPA 79? (see below)
NFPA 79
9.1.4 Connection of Control Circuit Devices.
9.1.4.1 All operating coils of electromechanical magnetic devices and
indicator lamps (or transformer primary windings for indicator
lamps) shall be directly connected to the same side of the control
circuit. All control circuit contacts shall be connected between the
coil and the other side of the control circuit.
A safety inspector is telling me the following circuit is in violation of the
above requirements:
115VAC Heating Element wired with a solid state relay (for circuit on/off, not
for temperature control) on the Neutral side of the heating element. There
are several thermal switches on the Line side of the heating element. The
relay is controlled by a 24 volt circuit and ground. There is a manual power
switch that opens both sides of the line. If the relay failed in a short
condition or if the neutral side of the heating element shorts to chassis
ground , no hazard occurs because the temperature is controlled by two thermal
switches.
How would this circuit violate 9.1.4.1? The safety inspector is saying that
this section of the standard mandates the solid state relay must be on the
Line side of the heating element along with the thermal switches. Is he
correct?
Thank you for your help.
The Other Brian
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