Knocking Out Power Surges

Power surges cause hundreds of millions of dollars in damage each year, but with the right protection, most of this damage can be avoided. This starts with proper design and specification of surge protecting devices, as well as adherence to the appropriate codes and standards. The first line of protection is a surge diversion device at the main service disconnect.

12/01/2003


Power surges cause hundreds of millions of dollars in damage each year, but with the right protection, most of this damage can be avoided. This starts with proper design and specification of surge protecting devices, as well as adherence to the appropriate codes and standards.

The first line of protection is a surge diversion device at the main service disconnect. Voltage, at this point, can be below or above 1,000 volts. As defined in NEC Article 280, only devices known as surge arresters can be installed on the line side of the service disconnect. They are not allowed on the load side. Surge arresters shall be connected to each ungrounded conductor, and if installed on circuits of less than 1,000 volts, shall be listed for the purpose.

The second line of surge protection devices will deflect any residual spikes to harmless levels. These devices should be located on the load side of the service disconnect, in panelboards feeding outdoor feeders or branch circuits or in panelboards feeding microprocessor-type equipment or outdoor telephone, data, security and communication lines. Per NEC Article 285, only surge protective devices called transient voltage surge suppressors (TVSS) can be located on the load side of the AC electrical service, and throughout the facility, for power distribution systems of 600 volts or less.

Another important point to consider for connection of TVSS on the load side of the service disconnect is defined in NEC Article 230.71. Only six service disconnecting means are allowed for each service. Because of this, the TVSS disconnecting means is one of the six disconnects; the seventh disconnect is not allowed. NEC Article 285, and UL1449, 2nd Edition, require that the TVSS shall be marked with a short-circuit current rating (currently, this isn't required for surge arresters). Also, the TVSS shall not be installed at a point on the system where the available fault current is in excess of that rating.

When specifying TVSS, it is important to highlight that overcurrent protection shall allow both protection during high surge (kA) events and during temporary overvoltage conditions, as well as protection for small fault currents.

Surge current rating is another parameter of TVSS. IEEE recommends testing service entrance TVSS units to only 10 kA, but the majority of manufacturers suggest specifying 250 kA per phase surge current rating for service entrance TVSS' because of life expectancy. However, a 160 kA per phase surge current rating is appropriate for TVSS' located in distribution panelboards, and a 90-120 kA per phase surge current rating is appropriate for TVSS' located in branch circuit panelboards.

The routing and length of conductors connecting the TVSS is an important concern addressed in NEC Section 285. Connecting the TVSS with the shortest conductor length possible provides the most effective protection. A TVSS installed as an internal part of a panelboard provides the optimal design.

During the design process, if engineers apply all related standards and pay attention to the above items, an enhanced, surge-protected power distribution system can be achieved.



Important surge suppression codes and standards

National Electrical Code, Articles 280 and 285

IEEE Emerald Book

UL1449, 2nd Edition

IEEE C62.41

IEEE C62.45

NEMA LS-1

NFPA 780



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