SURGE PROTECTION 101 : A BRIEFING FOR THE LAYMAN
by Richard Kithil, National Lightning Safety Institute
Discharge current rating is expressed in kilo amperes/microseconds/phase. More is better.
- WHY SURGE PROTECTION?
Overvoltages may stress sensitive electrical/electronic equipment, interrupting operational continuity and/or causing fires. Overvoltage sources can be from direct and indirect lightning or from substation switching activities.
- ARE THERE DIFFERENCES BETWEEN “ARRESTORS” OR “SURGE PROTECTION DEVICES (SPDs)”? The authority on this topic is the National Electrical Code (NEC). Arrestors are installed on circuits over 1 kV (NEC Section 280). SPDs are installed on circuits under 1 kV (NEC Section 285).
- HOW DOES SURGE PROTECTION WORK?
SPDs recognize overvoltages and react at very high speed. Fuses and breakers cannot respond this quickly. SPD technology clips or “knocks down” overvoltages so connected equipment will survive and will continue operating. SPDs also divert overvoltages to ground. “Grounds” must be designed and maintained so as to be low impedance destinations for these surges.
- GENERALLY, WHERE ARE SPDs APPROVED BY NEC FOR INSTALLATION?
SPDs can be installed on the supply side of the service or the electrical disconnecting means. They also can be installed on the load side. Formerly, NEC referred to them as “transient voltage surge suppressors or TVSS”.
- SPECIFICALLY, WHAT ARE THE SURGE PROTECTION REQUIREMENTS PER NFPA-780?
5.1 NFPA-780 (188.8.131.52): SPDs shall be installed at all power service entrances.
5.2 NFPA-780 (184.108.40.206): SPDs shall be installed at entrances of conductive communications systems (including but not limited to CATV, alarm and data) and antenna systems.
5.3 NFPA-780 (220.127.116.11): SPDs shall be installed at all points where an electrical or electronic system conductor leaves a structure to supply another structure if the conductors or cables are run over 30m (100 ft.).
5.4 NFPA-780 (18.104.22.168): SPDs shall be permitted for installation at subpanels or branch panels and at the point of utilization (outlet or signal termination; also termed supplementary protection).
5.5 NFPA-780 (22.214.171.124): SPDs shall not be required where, under engineering supervision, it is determined that surge threat is negligible or the lines are equivalently protected or where installation compromises safety.
- WHAT ARE RECOMMENDED THREAT LEVEL SPECIFICATIONS FOR SPD PROTECTION?