Purpose
MOVs are installed to prevent damage to sensitive electrical and electronic components from incoming service voltage spikes.
MOVs are used in one form or another on most all electronic devices. On PRIMUS sterilizers, these devices are located in the main electrical junction box.
Overview
While an infrequent problem in the field, it is worth noting that external voltage surges from the electrical utility grid supplying power to a facility, if not recognized and dealt with appropriately, can result in steam sterilizer issues.
The use of Metal Oxide Varistors (MOV) is a preventive measure that PRIMUS employs on all of its steam sterilizers.
Areas of the country with frequent lighting storms and/or unreliable electrical power are particularly susceptible to voltage spiking.
Technical Definition – Varistor
A varistor is an electronic component with a significant nonlinear current–voltage characteristic. Varistors are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from sensitive components. A varistor’s function is to conduct significantly increased current when voltage is excessive.
Metal Oxide Varistor
The most common type of varistor is the Metal Oxide Varistor (MOV). They contain a ceramic mass of zinc oxide grains in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbor forms a diode junction which allows current to flow in only one direction.
When a small or moderate voltage is applied across the electrodes, only a tiny current flows. When a large voltage is applied, the diode junction breaks down, and a large current flows. The MOV has a high resistance at low voltages and a low resistance at high voltages.
Voltage Surges from the Power Grid
Varistors can absorb part of a surge.
An MOV is designed to conduct significant power for very short durations (~8/20 microseconds), such as caused by utility voltage fluctuations, but typically does not have the capacity to conduct sustained energy. Under normal utility voltage conditions, this is not a problem. However, certain types of faults on the utility power grid can result in sustained over-voltage conditions. Examples include a loss of a neutral conductor or shorted lines on the high voltage system.
Limitations
Varistors do not absorb a significant percentage of a lightning strike as energy that must be conducted elsewhere is many orders of magnitude greater than what is absorbed by the small device.
If a transient pulse is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This catastrophic failure occurs when the manufacturer’s “Absolute Maximum Ratings” are significantly exceeded.
A varistor provides no equipment protection from inrush current surges (during equipment startup), from over current (created by a short circuit), or from voltage sags (also known as a brownout). A varistor neither senses nor controls such events. Protection from these power disturbances is normally provided by external devices such as a UPS.
Varistor Degradation
Varistor degradation is defined by manufacturer’s life expectancy charts using curves that relate current, time, and number of transient pulses. A fully degraded varistor remains functional (no catastrophic failure) and is not visibly damaged.
Have a question or having difficulties? Talk to Dave Schall, our PRIMUS sterilizer expert who is here to assist you anytime at 877.679.7800 extension 1212 or email at dschall@spire-is.com.
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