The performance of a shunt connected (in parallel with load) surge protection device is highly dependent on the installation practices used. The residual voltage of all shunt connected surge protection devices is measured directly at the terminals of the device during testing. This gives a relative gauge of performance, but does not reflect the device’s installed performance.
The length of wire used to connect the surge diverter to the power feed needs to be kept as short as physically possible during installation to reduce the voltage drop on either side of the diverter. The resistance of the cable generates a small part of this voltage drop, but the majority is caused by the inductance of the cable (between 0.5-1.5uH per meter).
The voltage drop due to this inductance can be calculated using the following formula:
The voltage drop across the cable equals the inductance multiplied by the change in current over time.
While the inductance of the cable is very small, during a surge the change in current over time is extremely large. The above example shows a relatively small 9kA surge with a change in current on the rising edge of 1kA/us or in SI units 1,000,000,000 Amps/second. For the above surge we see a voltage drop of 700V per meter of cable in addition to the SPDs residual voltage.
Keeping the connecting lead lengths as short as possible will ensure that the installation actually provides effective protection.
The performance of a surge protection scheme for a site is greatly enhanced when multiple levels of surge protection are used. Novaris recommends at least 2 levels of protection, especially for impulse category 1 equipment which is designed to withstand an impulse of only 1.5kV.
The first benefit of multiple surge protection levels is surge protection device coordination. When the high voltage of the surge is applied to the surge protection device it changes from a high impedance state to a low impedance state. The residual impedance combined with the current passing through the device, or the spark over voltage, determines the surge protector’s residual voltage. The point of entry device passes the majority of the surge current due to the impedance of the wire separating each level of surge protection devices. Because the secondary surge protection device passes less current, the residual voltage is less. This ensures the residual voltage will be less than the impulse category of the equipment.
The other benefit of multiple levels of protection is redundancy. If the point of entry protection device fails, the secondary protection device will keep the circuit protected. This means the equipment is not exposed while the primary protection is replaced.
Novaris only recommends the use of HRC fuses for overcurrent protection of their shunt (parallel) connected surge protection devices. Circuit breakers are designed with a coil that acts as an electromagnet. This switches the circuit breaker when the load current exceeds the threshold rating. The coil has a relatively large inductance which when exposed to the change in current during a surge, creates a substantial voltage drop.
Conversely, HRC fuses contain a flat strip of current limiting metal which melts when the current exceeds the threshold rating. This results in a level of inductance proportional to a wire conductor, the length of the fuse and its fitting. Novaris Type 2 SPDs have active alarm monitoring which monitors the state of the HRC fuse as well as the surge protection components.
The maximum fuse rating of a Novaris power surge protection device is provided in the technical datasheet and on the product label. It is recommended to select a fuse rating that is no higher than this maximum and 30% lower than the rating of the upstream breaker or fuse. This is to ensure that in the unlikely event of a surge protection failure, the surge protection fuse will blow and not the upstream breaker. This secures the site’s power delivery, even in the event of a surge protection device failure.