In the early days of metal oxide arresters during the mid to late 1970s, MOV blocks from all manufacturers exhibited some degree of ageing, whereby their power dissipation at any given voltage increased slowly, but continuously, over time. However leakage can still occur through imperfect end seals. Still, all have a charge content that, when passed through an arrester, results in a certain amount of energy absorbed by the blocks.
They are found on PCBs as those normally blue disc looking things paired up with a fuse to give some protection in the event of a short circuit. Rather, it is how this moisture interacts with internal surfaces and materials that becomes the issue. In these tests, sample blocks were subjected to MCOV for 1000h while maintaining block temperature at 115°C and it was considered that this was equivalent to 40 years service at 40°C. A. In better quality power socket strips you need to look for two lights, one for the power and the second for the indication that the surge device is OK or not.
DEHN has a range of PCB mounted MOVs called DEHNguard PCB for power electronic /electrical applications that address these aforementioned issues.
Developed for the power generation and power quality industries for invertor design and power supplies or filters etc. I have been a product manager , applications engineer and business/ marketing development manager for surge protection devices or SPDs for 20 years , working mostly with the surge and lightning standards BS EN 62305 and BS7671 amendment 1 on-wards.
This INMR article from 2013, contributed by industry expert, Michael Comber, discussed the modes by which most arresters fail.
The other part of the range is the plug element that goes into the base.
This implies that the arrester was: • not well designed, or List your company in the INMR BUYER’S GUIDE and LABORATORY GUIDE to increase your exposure!
One key component of a surge protector is a Metal Oxide Varistor (MOV) which is responsible for diverting the electricity. The base element, there are four of them, one for the L-PE and the other is N-PE. switching surges). The most common modern type of varistor is the metal-oxide varistor (MOV). The phase plugs are MOV based surge protection and the neutral plugs have a gas tube for suppression of the leakage current in the phase plug after an overvoltage stressing event on the MOV. A point is then reached where the incremental heat generated is in equilibrium with the heat the arrester dissipates into the surrounding air.
(Note: this scenario can be avoided by ensuring only non-hygroscopic collaring materials, such as glass, are used.) I was saw the three blue disc MOVs soldered across the L-PE, N-PE and L-N, but no short circuit or thermal protection. The accumulation of randomly oriented grains is electrically equivalent to a n… The burning issue of metal oxide varistors and how to stop thermal runaway.
they are a very popular solution to these MOV quirks. Ultimately, the heat generated reached a point where no stable operation could be maintained, as depicted by curve 3 in Fig. MOVs or Metal Oxide Varistors are a very popular product and widely used. Hopefully there is some interesting knowledge on SPDs and the application/ installation of these units into electrical systems, PV solar ,wind turbine and many other areas. They are primarily protective devices to dump the current in excessive over voltage events and so save the circuit board or end equipment. If the application is low current less than 16 amps then the DEHN VC280 2 would be a good solution as it is a fully functional SPD for PCB mounting with thermal protection and an alarm output in the form of a break contact circuit. Experiments show that differences in grain sizes and grain boundary characteristics cause nonuniform microstructure. What is the failure mode of a varistor?
But the above raises an important point, MOVs are very popular in appliances and equipment whether that appliance device is fixed or portable.
If you had a three phase invertor you could have four phase bases or three phase bases and the neutral base. due to a system disturbance, fault or switching operation), the MOV blocks conduct more current and begin to heat up. 3.
Independent Expert Technical Content Website for Power Engineers. After prolonged use they can degrade and the material that makes up the disc can start a leakage current due to internal damage after a surge event, this will create a higher leakage current over time and before you know it we have a thermal runaway event. 1. In yet other cases, a pinhole type failure can occur at the edge of the block, possibly causing material to be removed from its outside surface.
This situation could lead to a fire hazard.
The failure of an arrester almost always results in a complete short circuit inside its housing. Some of these may have very high amplitudes but relatively short duration (e.g. ... As the clamping voltage falls lower and lower, a possible failure mode is a partial or complete short circuit, when the clamping voltage falls below the protected line voltage. You have another option with the bases, each base can have a volt free changeover contact within them so you can tell when the MOV has failed, more on that later.
This ageing characteristic of blocks was recognized early on and was addressed in ANSI/IEEE as well as IEC test standards by means of accelerated ageing tests. As these are mostly used in power applications there is also a 120 minute TOV or temporary over voltage fail safe mode this is most likely in loss of neutral events. They comprise of two elements.
For the phase plug the values are 20kA nominal and 40kA maximum, the let through voltage is 1.5kV and the response time is less than 25 nanoseconds. Furthermore, against crowbar-type devices, varistor response time is typically less than a nanosecond, and devices can be built to withstand surges of up to a 70,000A surge. In this way you can configure the PCB base layout to be universal for both TN supplies and TT supplies. We specialize in insulators, surge arresters, bushings, cable accessories and more. In other cases, varistor blocks can be punctured in localized areas, either partially or completely through their body. following external line faults or switching) or lightning or switching surge overvoltages. For example, during the day suns heats the arrester such that the internal pressure increases relative to ambient and outward gas leakage occurs. These socket strips can be over loaded and that is a fire risk but the MOV design if fitted badly can be a cause of fires also. The varistor failure mode was found to be a characteristic of the current magnitude. INMR is an independent expert technical content website for power engineers, testing laboratories, utilities, and equipment manufacturers. This type contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides, such as small amounts of bismuth, cobalt, manganese oxides, sandwiched between two metal plates, which constitute the electrodes of the device. This implies that they become more rather than less thermally stable during service and therefore are unlikely to cause arrester failure due to ageing. Varistors need to absorb the energy deposited by temporary overvoltage, switching surges, or lightning impulses. For example, the resulting thermo-mechanical shock could cause them to crack into two or more pieces. If the input of energy is excessive, the resulting temperature rise of the blocks may be such that the arrester is pushed into a thermal runaway condition. All the bases are PCB mounted and accept a surge protection plug, so in a single phase power supply you would have one of each base and plug. Under normal operating conditions, which see the arrester energized at its maximum continuous operating voltage, Uc, the temperature of the MOV blocks rises only slightly above ambient. In the case of solid core arresters, moisture that has wicked into internal interfaces along either a portion or the complete length of the arrester can result in dielectric breakdown and failure. There is no down time with this pluggable design and if the volt free remote contact feature is used this change of state from OK to defective can be seen in real time and the reparative action carried out as soon as possible. If there is no LPS fitted then these units are OK for the incomer and that is useful for using them in new voltage optimiser applications. If the power frequency voltage across the arrester increases (e.g. In such a case (i.e. In most scenarios, failure occurs due to dielectric breakdown, whereby the internal structure has deteriorated to the point where the arrester is unable to withstand applied voltage, whether normal system voltage, temporary power frequency overvoltage (e.g. Ultimately, the complete stack can no longer withstand the applied voltage. This essentially shifts voltage to other blocks and leads to higher conducted currents, external to some blocks but internal to others. This can result in a fire, a quick look online at MOV Fires in a search engine will show the extent to which MOV failure has led to house fires and the finger normally gets pointed at the surge protection socket strip. If the devices are used within the characteristics of their performance data they are effective anti-surge components, however in all MOV applications there are some issues to be aware of.
If the temperature of the blocks is raised quickly (as a result of energy absorption) so that it becomes higher than depicted by the black dot on the same curve, then the arrester will not recover from this event and go into thermal runaway, as described earlier for a situation of prolonged TOV. The plugs are rated for 240v nominal voltage and 275v ac maximum but other types are available in the RS extended range, With MOV devices the parameters that are most informative are the discharge current (kA), voltage let through (Up) and response time (ns). Looking inside many different appliances I find MOVs soldered or strapped across the main terminals or connections without a fuse or thermal disconnection device of any kind. In this way the appliance is unprotected for as short a time as possible but was always on line. As long as the overvoltage is below some critical limit, a new thermally stable operating point will be reached, albeit at a higher MOV block operating temperature, as depicted by the green dot on curve 2.