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Selecting Protection - Power



Power line surge protection must:

1. Provide adequate protection for all equipment.

2. Achieve a long working life.

3. Optimise the cost and size of the surge protection devices (SPDs).


Options for Surge Protection Devices

There are two common configurations of SPDs:

One port SPDs are parallel or shunt connected across the line. These include the Novaris SD, SG and HSG products.


Two port SPDs are connected in series with the line. These include the Novaris SSP, SF and PP products.



There are two classes of SPD components:

Voltage limiting SPDs include metal oxide varistors and suppressor diodes. These have a high impedance when no surge is present but can reduce impedance continuously with increased surge current and voltage. These are also called “clamping devices”. Novaris SD, SSP, SF and PP products are voltage clamping SPDs. 


Voltage switching SPDs include spark gaps, gas discharge tubes, thyristors and triacs. These have a high impedance when no surge is present but can have a sudden change to a low impedance in response to a voltage surge. These are also called “crowbar devices”. The Novaris SG products are voltage switching SPDs. 


Sometimes a combination of these components may be used. The Novaris HSG is an example of a combination SPD.


Selection of Surge Protection Devices


1. Surge Diverters, SD

All Novaris surge diverters with initial product code SD employ metal oxide varistor (MOV) voltage limiting components. These can be used for main switchboard primary protection, distribution board and final circuit protection. As voltage limiting components there is no follow on current, and with suitable fusing these are easy to install and operate. 



SD products are suitable for all applications except where extreme voltage fluctuations may be experienced. Excessive overvoltage can damage MOV based SPDs although all Novaris surge diverters are housed in metal enclosures and meet the fail-safe requirements of UL1449 - specifically package rupture and the effects of excessive heating. 



Novaris manufactures surge diverters to suit all applications from high exposure environments to final circuit protection with ratings of Imax up to 250kA (8/20μs) or Iimpof 25kA (10/350μs)*. 



Like all one port shunt connected SPDs, performance can be compromised by the presence of long connecting leads, particularly in physically large main switchboards. For this reason primary SPDs on main switchboards would be followed by secondary protection on distribution boards and final circuits.


2. Spark Gaps, SG

Spark gaps have high surge ratings and are suitable for point of entry protection in installations with highly exposed overhead LV power lines with no local transformer in high lightning areas. As voltage switching SPDs, spark gaps have a crowbar effect and effectively place a short circuit across the line once fired. Thus high levels of AC follow on current will flow. Unless properly configured to be compatible with the AC fault rating of the supply and suitably fused, spark gaps can cause nuisance tripping of supply circuit breakers and extreme voltage disturbances whilst the follow on current flows. 


Novaris spark gap SPDs have surge ratings (Iimp) of up to 110kA (10/350μs). Triggered spark gaps must be followed by secondary protection further downstream in the distribution network because they have a high impulse firing voltage.


3. Hybrid Spark Gaps, HSG

Hybrid spark gaps combine the best qualities of voltage switching and voltage limiting components. Novaris HSG hybrid spark gaps are suitable for all high exposure installations and meet the recommendations of IEC61643-12 in relation to surge ratings with Imax of 250kA (8/20μs) or Iimp of 25kA (10/350μs)*. The spark gap in the HSG is a high energy gas discharge tube with a clearly defined impulse firing voltage, its let through voltage closely approaches that of an MOV based surge diverter.


The hybrid combination ensures that there is no follow on current and the HSG may be as easily deployed as our SD range. The HSG is able to tolerate excessive temporary overvoltages (TOV) and is ideal for applications where mains voltages fluctuations are significant.


4. Series Surge Protector, SSP

All shunt connected SPDs are compromised in performance by the presence of their interconnecting leads. Typically voltage drops of 500V per meter of connecting lead can be expected. Such lead lengths are often unavoidable in physically large main switchboards. Nevertheless one port SPDs provide effective protection for the main switchboard. 


For circuits that are more sensitive the SSP provides a means of eliminating the shunt connected leads and places the SPD directly across the line. Such applications might include UPS inputs, rectifiers, VSDs and motors.


5. Surge Filters, SF

The surge filter is a true two port SPD offering an extremely low let through voltage capable of protecting the most sensitive of electronic circuits. The Novaris range of surge filters is extensive: from 2A DIN rail mount units designed to protect sensitive PLCs and process equipment; plug in units for final circuit outlets; to 2000A per phase filters designed to protect major data centres. 


Surge ratings up to 250kA (8/20μs) are available making surge filters suitable for providing primary and secondary protection in one package as may be required at a cellular basestations, process plant control rooms or data centres. As surge filters are series connected they must have a current rating IL equal to or greater than the protected circuit.



*Surge Ratings: tests conducted by some manufacturers and informally reported to the IEEE have indicated that the stress imposed on an MOV based SPD by a 10/350μs impulse might be equivalent to the stress imposed by a standard 8/20μs impulse, with a scaling factor of 10. Thus an SPD with Iimp=25kA could be equivalent to Imax= 250kA. From IEEE Std C62.41.2-2002