AC protection

Surge protection should be installed at the AC input to the weighing computer, as the system can be damaged via transient voltages arriving via the power utility circuit. Damaging transient voltages on the power circuit can occur due to a direct lightning strike to the power circuit or a nearby strike, which iscoupled magnetically or capacitively.
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However, it is estimated that approximately 85 percent of all transient voltages are generated internally within one’s own facility by the switching of electrical equipment. Most of these transients are generated through such events as:

  • Switching of inductive loads.

  • Switching of power factor correction capacitors in distribution systems.

  • Arcing of fuses or circuit breakers clearing power faults.

  • Conducted electromagnetic interference from power switching electronics such as thyristor drives and switch mode power supplies.

In an attempt to overcome the many extravagant claims made by some manufacturers of SPDs, the National Electrical Manufacturers Association (NEMA) has sought to clarify and normalize the way in which these devices are specified. NEMA LS1 is a voluntary guide for manufacturers, which describes the relevant performance criteria of SPDs. It is Erico’s policy to follow this guideline.

It is also important to note that UL does not test products to the manufacturer’s claimed surge rating. Some manufacturers are known to claim ratings well in excess of what can be achieved in practice. These claims are often based on the aggregate number and size of metal oxide varistors alone. They do not mention the fact that internal series disconnecting devices may activate well before this claimed rating. (Permanently connected TVSS’s are only tested by UL to a maximum rating of 10kA 8/20µs.)

In 1998, UL 1449 Edition 2 was released. This safety standard included a number of significant improvements, particularly in the area of personnel safety pertaining to the risk of electric shock and SPD-initiated fire. A new testing regime calls for the product to meet designated over-voltage conditions for prescribed periods of time without the risk of fire, ignition, charring, emission of flame or molten metal, or enclosure disintegration. UL’s motivation behind this change was a growing concern that SPDs may pose a fire hazard under conditions of sustained over-voltage. This can occur when the utility supply is poorly regulated or when the neutral is inadvertently disconnected in an unbalanced distribution system.

Most solutions to address UL 1449 Edition 2 use internal fuses or thermal disconnects to permanently isolate the MOVs during a sustained over-voltage and prevent a fire. However, this approach can limit the surge handling capacity of the device and reduce its useful life because the SPD will need to be replaced. Erico developed Transient Discriminatingª technology that incorporates a frequency discriminating switch. This safely eliminates the risk of fire during sustained over-voltage events while maintaining continued protection after the event. This quantum leap in technology adds a level of “intelligence” to the SPD, enabling it to discriminate between sustained over-voltage conditions and true transient voltage events. Not only does this ensure safe operation
under practical application, but it also prolongs the life of the SPD since no permanent disconnects are used.
Electronic equipment is sensitive not only to the absolute magnitude of the voltage, but also to the rate of rise of this transient. The inclusion of a low-pass filter stage helps condition the wave shape, thereby providing superior protection for sensitive electronics. It is important to realize that a number of different topologies of filter circuits exist, each providing significantly different performance. At its simplest, a manufacturer may include a capacitor in parallel with the output. This will serve to reduce any fast ringing voltages and will also absorb the energy in a small transient, thereby providing a level of attenuation. A true in-line low-pass filter can reduce the dv/dt from a nominal 10,000V/µs to less than 100V/µs, if the maximum attenuation occurs across the band of frequencies where most of the surge energy spectrum is centered (typically 25-50kHz). Low cost, parallel filters are largely ineffectual at such frequencies and are really only suited to filtering at EMI/RFI frequencies.

For AC services, SPDs should be installed in a cascaded fashion. IEEE C62.41 details the need for primary protection, where the aim is to arrest the large externally generated surge before it can enter the internal environs of the facility. Also, by limiting this energy at the point-of-entry, we can prevent coupling onto adjacent wiring and circuits within the building. Competition between manufacturers of SPDs has seen ever-increasing surge ratings being offered for this role. The situation has reached the ridiculous, where the surge ratings being specified will never be required, and where theoretical life expectancy exceeds hundreds of years. Erico maintains that a single shot rating of 100kA 8/20µs per line is sufficient for all practical purposes.