To help you learn more about how the requirements for protection of overvoltages due to lightning and switching surges of safety circuits have been revised according to the 2021 edition of the National Fire Protection Association (NFPA) 79, UL Solutions and Eaton Bussmann division experts have teamed up to provide answers to the following frequently asked questions from our related webinar.
Is additional surge protection required if it’s included in equipment that provides a safety function?
Typically, most equipment that employs sensitive electronic circuits is provided with some type of surge protection. This surge protection is usually a type 5 or type 4 component assembly. The type of surge protection employed in the equipment may not provide the desired level of protection. The protection needed can vary depending on the conditions of the power supply to the equipment and the location of the equipment in the circuit and distance from the supply. If the power supply is known to be susceptible to power line surges and surges due to lightning strikes are a common problem, the surge protection provided with the equipment may not provide suitable protection. If the power supply is void of these conditions or the equipment is located considerably downstream from where the supply is located, the surge protection provided with the equipment may provide suitable protection. When designing a circuit that includes sensitive electronic equipment, especially circuits for an industrial control panel, it’s best to consult the manufacturer of the equipment in order to determine if the surge protection included as part of the equipment will provide the protection required.
Does the surge protective device (SPD) need to be on 120 volts alternating current (VAC) line voltage or on the 24 volt direct current (VDC) system that powers the safety circuit?
In general, the SPD should be applied as close to the device that needs to be protected. So, if the device that is performing the safety function is in the 24VDC side then that is where the SPD should be located. However, it is not uncommon to apply SPDs upstream of this SPD to provide increased protection.
Is surge protection required for all industrial control panels on an industrial machine?
No, surge protection is only required for industrial control panels that supply safety circuits on the industrial machine. The surge protection devices may be installed in the control circuit and/or power circuit depending upon the specific design and construction of the safety circuit.
Where the SPD is supplied with conductors for phase and neutral conductors, should these conductors be reduced in length if possible?
Yes, to ensure the best performance and protection of the SPD, it is critical to keep the length of the conductors from the SPD as short as possible and avoid 90-degree bends where possible.
What is the most important rating for SPDs?
Multiple important ratings exist for SPDs. From a safety perspective, it is very important that the nominal system voltage rating and short-circuit current rating (SCCR) are properly selected for the applied system. From a performance and protection perspective, the nominal discharge current (In), maximum continuous operating voltage (MCOV) and voltage protection rating (VPR) are the most important ratings.
