The installation of SPDs is often poorly understood. A good SPD, incorrectly installed, can prove of little benefit in real-life surge conditions. The very high rate-of-change of current, typical of a surge transient, will develop significant volt drops on the leads connecting the SPD to the panel or equipment being protected. This can mean higher than desired voltages reaching the equipment during such a surge condition. Prosurge suggests that measures to counteract this effect include locating the SPD so as to keep interconnecting lead lengths as short as possible, twisting these leads together. Using a heavier gauge AWG cable helps to some extent but this is only a second order effect. It is also important to keep protected and unprotected circuits and leads separate to avoid cross coupling of transient energy.

This is a difficult question and depends on many aspects including – site exposure, regional is okeraunic levels and utility supply. A statistical study of lightning strike probability reveals that the average lightning discharge is between 30 and 40kA, while only 10% of lightning discharges exceed 100kA. Given that a strike to a transmission feeder is likely to share the total current received into anumber of distribution paths, the reality of the surge current entering a facility can be very much less than that of the lightning strike which precipitate it.

The ANSI/IEEE C62.41.1-2002 standard seeks to characterize the electrical environment at different locations throughout a facility. It defines the service entrance location as between a B and C environment, meaning that surge currents up to 10kA 8/20 can be experienced in such locations. This said, SPDs located in such environments are often rated above such levels to provide a suitable operating life expectancy, 100kA/phase being typical.

Although often used as separate terms in the surge industry, Transients and Surges are the same phenomenon. Transients and Surges can be current, voltage, or both and can have peak values in excess of 10kA or 10kV. They are typically of very short duration (usually >10 µs & <1 ms), with a waveform that has a very rapid rise to the peak and then falls off at a much slower rate. Transients and Surges can be caused by external sources such as lightning or a short circuit, or from internal sources such as Contactor switching, Variable Speed Drives, Capacitor switching, etc.

Temporary over voltages (TOVs) are oscillatory phase-to-ground or phase-to-phase over voltages that can last as little as a few seconds or as long as several minutes. Sources of TOV’s include fault reclosing, load switching, ground impedance shifts,single-phase faults and ferroresonance effects to name a few. Due to their potentially high voltage and long duration, TOV’s can be very detrimental to MOV-based SPD’s. An extended TOV can cause permanent damage to an SPD and render the unit inoperable. Note that while UL 1449 (3rd Edition) ensures that the SPD will not create a safety hazard under these conditions, SPDs […]

A direct lighting strike is the most powerful and difficult surge to protect against. Prosurge recommend that Proper grounding and bonding of the electrical system and employing proper surge protection can protect sensitive equipment. A SPD with a higher single surge current rating will perform best against this type of event, if the unit is properly installed and the grounding system is adequate.The maximum single withstand surge current rating is defined in IEEE SPD Standard C62.62.

Some key differences between Type 1 and Type 2 SPDs are:

• External Overcurrent Protection. Type 2 SPDs may require external overcurrent protection or it may be included within the SPD. Type 1 SPDs generally include overcurrent protection within the SPD or other means to satisfy the requirements of the standard; thus, Type 1 SPDs and Type 2 SPDs that do not require external overcurrent protection devices eliminate the potential for installing an incorrectly rated (mismatched) overcurrent protection device with the SPD.
• Nominal Discharge Current Ratings. Available Nominal Discharge Current (In) ratings of Type 1 SPDs are 10 kA or 20 kA; whereas, Type 2 SPDs may have 3 kA , 5 kA, 10 kA or 20 kA Nominal Discharge Current ratings.
• UL 1283 EMI/RFI Filtering. Some UL 1449 Listed SPDs include filter circuits that have been evaluated as a UL 1283 (Standard for Electromagnetic Interference Filters) filter. These are complimentary UL Listed as a UL 1283 filter and a UL 1449 SPD. By definition and the scope of UL 1283, UL 1283 Listed filters are evaluated for load-side applications only, not line-side applications. Consequently, UL will not complimentary list a Type 1 SPD as a UL 1283 Listed […]

Type 1 SPDs (Listed) – Permanently connected, hard-wired SPDs intended for installation between the secondary of the service transformer and the line side of the main service equipment overcurrent protective device, as well as the load side of the main service equipment (i.e. Type 1’s can be installed anywhere within the distribution system). Type 1 SPDs include watt-hour meter socket enclosure type SPDs. Being on the line side of the service disconnect where there are no overcurrent protective devices to protect an SPD, Type 1 SPDs must be listed without the use of an external overcurrent protective device. The Nominal Discharge Current Rating for Type 1 SPDs is either 10kA or 20kA.

Type 2 SPDs (Listed) – Permanently connected, hard-wired SPDs intended for installation on the load side of the main service equipment overcurrent protective device. These SPDs may also be installed at the main service equipment, but must be installed on the load side of the main service overcurrent protective device. Type 2 SPDs may or may not require an overcurrent protection device per their NRTL listing. If a specific overcurrent protection is required, the SPD’s NRTL listing file and labeling/instructions are required to note the size and […]