Brian, Unfortunately, you inspector is incorrect, although you are probably going to have an issue with convincing him of this. Section 9.1.4.1 specifically says "All operating coils of electromechanical magnetic devices", it makes no reference to the load side of the device. It also makes no mention of non-electromagnetic devices.
You probably alrea dy know this, but for clarity I think it needs mentioning: The practice of connecting one side of of the control device coil to the grounded current carrying conductor (neutral) is simply to ensure that a ground fault on the relay coil to the chassis of the equipment (presumably soundly bonded to the protective grounding conductor) either has no effect (neutral coil terminal to chassis ground) or immediately blows the control circuit fuse (line terminal of the coil to chassis). If the control devices were connected in the neutral side with the line coil terminal connected directly to the control supply, then a ground fault on the neutral would cause loss of control of the load. None of the above applies to the load side of the control device, since the clause speaks only to the "operating coils". Normally the control device is always connected in the ungrounded current carrying conductors, unless both sides of the load are switched by the same device. I went searching for a specific section supporting this in the NEC, NFPA 79 and UL 508A and could not locate it, although I'm sure it's there somewhere. The intent is the same - the protective grounding conductor is not intended to carry current except in the case of a fault, and then only long enough to enable the circuit protection to open the circuits. Doing otherwise can lead to unacceptable touch currents on the chassis of the equipment. If a ground fault occurred on the neutral side of the SSR, you would still have control but you might be forcing current into the chassis and protective grounding system. If it happens on the line side of the SSR, you lose control of the load and you may force current into the chassis resul ting the aforementioned touch currents. My guess is that the inspector cited 9.1.4.1 because, like me, he couldn't find the section in the NEC or UL 508A that prohibits switching the neutral with a control device. Anyone who could cite that section of the code would probably have the answer. -- Doug Nix, A.Sc.T. IEEE PSES Kitchener-Waterloo Section, Ontario, Canada [email protected] mobile (519) 729-5704 fax (519) 653-1318 Find me LinkedIn at http://www.linkedin.com/in/dougnix On Oct 27, 2008, at 12:53, Kunde, Brian wrote: 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 _________________________ LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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