Lightning has been the subject of research for decades. Extensive research has been conducted by the University of Queensland in Australia on the failure mode of surge protectors during multiple strokes. They have created a multi-pulse (6 stroke) generator that is capable of providing a time interval of between 20 to 130 ms per pulse. This is to simulate the effects of multi-stroke lightning events which may destroy a surge protector.

This multi-pulse generator uses a series of capacitors that are connected to the pulse forming network by a bar switch which is connected to a pendulum. As the pendulum swings through the charged capacitors, it discharges the capacitors into a pulse-forming network. The time interval between switch contacts is a function of the period of the pendulum. It has been found that fairly short time intervals between pulses will have a severe effect on the performance of surge protectors and also may cause nuisance fuse operations.

The result of multi-pulse generator testing on surge protectors has shown that the edge of the zinc oxide metalized area on any MOV is the most critical region when subjected to this kind of pulsing. It was also found that in some cases the temperature can rise to between 800° and 1000° C in isolated “hot spots” in the MOV volume, resulting in failures from punctures of the zinc oxide material. Also, plasma accumulates on the edge of the metalized area and causes additional breakdown regions. The Australian team concluded from their research that the actual rating of an MOV device can be as much as 100 to 150% of the nominal rating for a single pulse; and 60 to 75% of the rating for multiple pulses, if the device is not energized; but may be between 30 to 40% of the rating for multiple pulses if the device is energized.

For more details about lightning read APT’s engineer bulletin: Lightning: Physics and Effects (pdf).

Following is generalized information about design choices influencing Surge Protective Device (SPD) safety.  As an overview, surge protector failures are caused by system level sustained overvoltages. Surge Protectors rarely fail from surges. When MOV(s) conduct a sustained overvoltage, they can overheat.  If heat is generated very, very quickly, the MOV(s) can rupture or explode.  At the opposite end of the spectrum, if heat can be transferred away, MOV(s) might warm up to tolerable levels.  If MOV(s) generate more heat than can be dissipated, thermal runaway results in overheating and a fire hazard. 

Overcurrent Protection (OCP) – OCP should be selected such that higher current MOV(s) faults are cleared prior to MOV(s) rupture.  This is easier said than done because OCP needs to pass large surge currents, while clear modest fault currents.  Because most surge protectors have multiple internal current paths, it is  important that OCP stays cleared after operation and does not inadvertently reclose. 

Thermal Disconnector – Any current drawn by MOV(s) below OCP’s operating threshold is not considered  over-current and passes through the MOV(s).  Thus OCP is blind to smaller fault currents.  During sustained MOV current draws, heat is generated via I²R losses.  Thermal disconnectors operate on heat, not current.  When MOV(s) overheat, the thermal disconnectors will clear them from the circuit as a safety precaution.

Coordinated OCP and Thermal Disconnectors - This ensures that the thermal protection of the thermal disconnector(s) overlaps OCP’s blind spot such that MOV(s) are satisfactorily protected.

MOV size – Larger MOVs tend to be more robust.  In addition to obvious durability advantages, the additional mass acts as a heat sink.  This can  yield an extra few moments needed for OCP to operate, whereas smaller MOV(s) might rupture sooner. 

Encapsulants – Encapsulants  can provide bi-directional containment, plus offer limited heat transfer capabilities.  These assist by containing internal ruptures and preventing outside contaminants from reaching key components.  Common encapsulants are epoxy based or sand.(Unencapsulated surge protectors are usually associated with lower-end products.)   Epoxy based encapsulants are good insulators and offer better mechanical and vibration isolation than sand.  If overheated above about 450°F, epoxies tend  to liquefy.  Some epoxies can be  brittle.  APT developed and patented Ceramgard as an elastomeric compound that solves this issue.  Sand is inorganic and a good insulator, with manufacturing challenges because of  its leaky and nonhardening nature.  Encapsulants can transfer limited heat to their enclosures.

For more information about design choices influencing Surge Protective Device (SPD) safety, please read the rest of the this white paper on our website.