Zonemaster: State-of-the Art Technology Improves Surge Performance and Reduces Size Wiremold’s ZoneMaster Series protectors were designed to solve the problems inherent in traditional surge protection design. Panel mounted Surge Protection Devices (SPD’s) typically consist of surge protection elements in some enclosure, either with or without fuses. The inclusion or exclusion of fusing has to date been a design compromise, a choice between safety and performance. A number of independent research papers have shown conclusively that fuses (with ratings below 100A) can significantly limit the surge capacity of an SPD. Conversely, the lack of fusing can represent a safety hazard to the installation. The design also allows very fast, precise control of transient voltages without resorting to silicon avalanche diodes (SAD’s). The Patented Technology Wiremold’s revolutionary ZoneMaster product design now allows the manufacture of a protection module 1” x 2” x 4”, that incorporates large block MOVs and allows surge currents of 150,000A 8/20 µs to flow without premature fuse operation. The success of the patented design revolves around massive, custom, three terminal MOV assemblies, each with a surface area in excess of 10,000mm, interconnected with a fuse link via a low melting point eutectic alloy. The three terminal MOV’s facilitate extremely low clamping voltages, by achieving a lower on state impedance than individual MOV components. Surge current entering on the center electrode is divided through the metal oxide material equally, exiting the component via two outer electrodes. Such perfect sharing is impossible with discrete off the shelf components. Two of these assemblies are incorporated into one module, giving a theoretical maximum surge current rating of 180,000A (sum of components). This is not how the device is rated. Simply adding the individual ratings of nonlinear components can be highly misleading; indeed extensive testing shows that 150,000A is the actual achievable rating per module. Internal Fuselink Operation The fuselink is the result of many hours of testing and computer simulation engineered to provide a careful balance between maximizing the surge handling capability of the protector and minimizing the time to disconnect a faulted MOV block from service. Independent testing of the fuselink has shown that the fuse does not prematurely operate under surge conditions. Surge tests at the full rated surge current truly demonstrate the capacity of the fuse MOV combination. Tests performed at the Kearney Electric Research Laboratory and Kinectrics Laboratory show that the design will withstand the full rated test current of 150,000A without performance degradation. The low melting point eutectic alloy directly connects the body of the block MOV to the fuselink, the whole assembly being held under tension by a spring. Should an abnormal supply condition cause thermal runaway in a block MOV, the alloy melts and the fuselink springs back, thereby disconnecting the faulty component and preventing excessive heat buildup. Extensive onboard and remote indication keeps users informed of protector status at all times. Key Performance Benefits Massive three terminal block MOV’s provide lower and faster surge response by controlling parasitic impedance. The combination of block MOV and thermal fuselink allows control of 150,000A + surge currents. Fuselink disconnects ONLY faulty components, providing protection redundancy. Status indication is mechanically linked to the fuse mechanism, correct monitoring is thus independent of AC power. Understanding Surge Protection Design Flaws Exposing the FUSING Myth It is a common mistake or misunderstanding to assume that fuses designed to interrupt AC faults will not operate on the short duration surge currents that originate from lightning. Fuses operate on a predictable I2t characteristic. For a low level AC power fault the fuse may take a second to operate; however, if “I” (current) is large enough, the fuse will operate in microseconds. I2t is sometimes referred to as the joule integral or specific energy. A number of papers (ref) have been published to examine the response of fuses to surge currents. All come to the same conclusion: the importance of fuse operation on surge protector performance is significant. For example, an SPD theoretically rated at 350,000A, incorporating quality 35A fuses, can only reliably discharge surge currents up to 18,000A without fuse operation. Once the fuse has cleared, the SPD is off line and the installation is unprotected. Clearly, lack of customer knowledge in this area has led to a proliferation of products with high surge current ratings that were determined by testing with fuses removed, or not tested at all. Why incorporate fuses if they cause so many problems? Metal Oxide Varistors, like any other electronic component, can and do fail. The fact that a correctly specified component can provide 15 to 20 years of service, does not eliminate the need to analyze and design for failure. MOV’s fail in two fundamental ways; hard short circuit or thermal runaway. The short circuit failure occurs when the component reaches the end of its useful life or is overstressed in the presence of a power source capable of delivering high fault current. Overcurrent protection is thus necessary to limit the fault current before the MOV literally explodes. Without appropriate fusing a faulted MOV can produce substantial explosive force, smoke, flame and white hot metal oxide material. Users should be aware that the fault current available at the service entrance of a commercial facility can approach 100,000A; sufficient energy to produce impressive fireworks. The second failure mechanism, thermal runaway, can occur prior to the MOV shorting, for the same reasons. However, certain abnormal power conditions, such as loss of neutral, can limit the fault current to a level below that required to operate fusing. As a result the MOV simply “cooks” getting hotter and hotter with no protection device able to intervene. Here a thermally operated fuse or disconnect adds considerably to the safety of the device without adding to the cost. Some surge protective devices employ SAD technology to achieve a low transient control level. SAD’s, while providing a low transient control level, are very limited in their surge current capacity. Some panel mount SPD’s employ both MOV’s and SAD’s. With this design it is almost impossible (without the use of large series inductors, which are not practical in panel mount SPD’s) to achieve a high level of surge capacity. 20mm Diameter MOV’s vs. Block MOV’s Manufacturers of main panel SPD’s employ either a number of 20mm MOV’s in parallel (the same size as is used in many receptacle suppressors) or larger block MOV’s. The 20mm devices have one advantage; they are cheap, cents compared to dollars for block devices. Problems can arise, however, in ensuring that each parallel device shares the surge current equally. In spite of attempts to match components, the non linear nature of MOV’s suggests that these small devices will never share the surge current equally. It is a fact that surge current sharing will be most unequal at lower magnitudes where the MOV is operating in the flattest (most non-linear) region of the V-I characteristic. If the surge current is of a longer duration than the standardized 8/20 µs (say 200 to 1000 µs) then the 20mm MOV that carries the lion’s share of current will degrade prematurely. Several independent papers have shown that SPD’s incorporating large diameter block MOV’s are to be preferred at service entrance locations. Large diameter MOV’s offer greater reliability and stability when subjected to a wide range of surge magnitudes and duration. Wiremold’s three terminal large block MOV’s offer additional advantages of lower parasitic impedance and, therefore, better transient control and higher surge capacity.