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 I<sup>2</sup>R 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.