Surge Protection Schemes Not Just for TVSS In the Specifier's Notebook "Knocking Out Power Surges," (CSE 12/03 p. 62), by Alan Mayzenberg, the author notes, "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.
Surge Protection Schemes Not Just for TVSS
In the Specifier's Notebook "Knocking Out Power Surges," (CSE 12/03 p. 62), by Alan Mayzenberg, the author notes, "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."
For the record, Article 280.22 is titled "Installed on the Load Side of Less Than 1,000 Volts." I believe that this may be interpreted as allowing the installation of secondary surge arresters on the load of the service disconnects.
Again, quoting from the article, "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."
It is my belief that Article 285 limits the installation of TVSS devices to the load side of the service disconnect. This is in agreement with UL Standard 1449 that states that UL1449-listed devices "are intended for installation on the load side of the main overcurrent protection." I cannot find any statement in Article 285 that limits the use of surge protection devices on the load side to TVSS only.
Peter W. Lewis, Intermatic, Spring Grove, IL
Alan Mayzenberg responds:
Both comments relate to the application of TVSS and surge arrestors (SA). The objective of SA is to ensure that surges do not exceed the basic insulation level (BIL) of the conductor wires, transformers, panelboards and other electrical equipment, and to protect the electrical distribution system and not the sensitive solid-state equipment from the effects of lightning.
SA's have a high let-through voltage, which is a key performance factor for protecting electronic loads. Under the IEEE category C3 test wave, the let-through voltage is typically more than 1,200 volts on a 120-volt AC system. Surges can also enter through branch circuits or feeders external to the building. Equipment within the facility that switches on and off can create surges, as well.
Using TVSS at the service entrance and the panelboards serving sensitive electronic equipment, the surge will be effectively reduced to under 100 volts. This cascaded approach is recommended by the IEEE Emerald Book.
NEC article 285 and UL1449 requires that TVSS cannot be installed before the main service disconnect, but they can be installed elsewhere within the facility after an overcurrent protection device. This is indicated in article 285.21, paragraph (A)(1), (A)(2), (A)(3), and in NEC 2002 Handbook explanations to paragraph 285.6.
TVSS are tested and marked with a short-circuit rating. SA are not tested and marked with a short-circuit rating. That, in my opinion, contradicts NEC paragraph 110.10, which requires that the circuit component short circuit ratings be selected and coordinated to permit the circuit protective device used to clear a fault to do so without extensive damage to the electrical components of the circuit. This can present a safety problem and issues with the authority having jurisdiction. Also, IEEE C62.11-1993 has been replaced by IEEE C62.11-1999 for devices rated greater than 1,000 volts and IEEE C62.34 for devices rated 1,000 volts or less (named as Secondary Surge Arresters). So evaluation of surge arresters according to IEEE C62.34 and UL1449 rather than to IEEE C62.11 will be needed.
However, the UL standard 96A for lightning protection systems requires that only a surge arrester be installed at the AC service entrance to protect the wiring and electrical equipment. Dual-listed devices, approved as TVSS and as SA, can be used for this case.