The burning issue of metal oxide varistors and how to stop thermal runaway

MOVs or Metal Oxide Varistors are a very popular product and widely used. They are primarily protective devices to dump the current in excessive over voltage events and so save the circuit board or end equipment.

The events that they are used to mitigate against are the normal everyday surges from man-made units like inductive loads and switching events that may cause processors to crash or LEDs to fail prematurely.

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. 

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.

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.

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. 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.

I had one of these cheap surge trailing socket strips from a high street store and cracked it open to see what was inside.

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 BS standards in socket strip design has no reference to the MOV inside, my socket strip had no neon/ indicator light either so I could not tell if the power was on or not and I could not tell if the MOV was OK.

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.

But the above raises an important point, MOVs are very popular in appliances and equipment whether that appliance device is fixed or portable.

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.

All MOVs have the same reaction to over stressed events and those that do not have a protective device associated with them are a fire risk.

Also MOVs could fail as a short circuit not in open circuit mode so we have two situations in MOV applications that need addressing by a designer.

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. they are a very popular solution to these MOV quirks.

The range is in the surge protection part of the RS product range not in the MOV /passives area. They comprise of two elements.

The base element, there are four of them, one for the L-PE and the other is N-PE. 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. 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.

If you had a three phase invertor you could have four phase bases or three phase bases and the neutral base. In this way you can configure the PCB base layout to be universal for both TN supplies and TT supplies. This layout is found in the wiring regs BS7671 in section 534 the layouts are seen as CT1 and CT2 and stems from a safety issue when MOVs are used with RCDs and that leakage current issue again.

The other part of the range is the plug element that goes into the base. They are phase or neutral plugs and can only plug in the correct base so there is no confusion.

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.

All the plugs however solve the MOV problem of thermal runaway as they have a thermal disconnect unit in them.

Yes this is not a unique feature but if the standard PCB mounted MOV disc does fail with a thermal event it may result in the board needing to be replaced or swapped out to replace the defective MOV component and fuse if that has also blown can be replaced as well.

With the DEHNguard PCB design the MOV plug is simply removed from the base, and this is also OK if the appliance is on as the phase and neutral plug is across the supply and not in series.

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. In this way the appliance is unprotected for as short a time as possible but was always on line.

If you have no need for the remote option or no way to use it , no matter as each plug has an indicator window that shows that the plug is OK or not.

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)

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.

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. 

This type of surge device is a type 2 unit so it is suitable for most applications in where there is a type 1 or lightning SPD at the incomer if the building has a fitted LPS or lightning protection system.

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.

There is also a fuse rating in much larger current applications to protect against failure by short circuit , this value is 125 amp so any application or appliance less than 125 amps needs no extra fusing in the surge circuit, the failure mode of the plugs are open circuit.

To assist the PCB designer the bases have 3D STEP files this shows the pin layout for the basic base and the volt free change over contact if selected.

Full datasheets and installer guides are also available.

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.