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Phase failure relays: design and installation

21 November 2012

When is a phase failure relay not a phase failure relay? When it’s failed of course, says Malcolm Greenhill. Phase failure relays (P

How many electronic devices in your control panels operate on 400/415Vac? Would that be just one? A PFR is probably the only electronic device in a control panel that is powered by the incoming three phase supply, while internally they only use low voltage. This means that they must dissipate a lot of electrical energy and one way they do this is to get warm.

This is allowed for in the design process, where capacitors are used to drop the voltage. Better types use capacitors with internal series connections, but the quality from different component manufacturers differ and only experience will weed out the bad ones.

This issue is compounded by the fact that in order to measure the supply voltage a PFR must have a low input resistance, whereas other supply powered devices will have a high input resistance, which effectively eliminates much of the external electrical noise, depending on the source.

Heat sources
Incorrect installation
- We all like to do a neat job and in many instances we see PFRs ‘crush’ mounted next to the motor contactor. This may look neat but the surface temperature of a contactor at nominal voltage can be in excess of 60oC. Add this source of heat to that generated internally by the PPR and the operating temperature for the device is immediately outside the temperature tolerance limits of its internal components. Ensuring that there is an air gap of up to 10mm between the contactor (or other adjacent heat source) and the PFR will help; any more than 10mm generally makes no great difference.

Harmonics - Harmonics on the supply cause increased current flow in the PFR’s power supply, and increased current equals increased heat. Many a site harmonic survey has revealed this to be the cause.

Spikes and surges - These cause gradual degradation of the foil capacitors in the PFR’s power supply by causing a flash-over between the foil ‘plates’. Even if self-repairing foil capacitors are used, these will still deteriorate over time. Add to that any heat from other sources, which will deplete the electrolyte in these capacitors, and the life of the PFR is seriously reduced.

A first indication that this is happening will be chatter from the output relay as it tries to pull in, indicating that the power supply is not delivering sufficient power. Of course, you could use a PFR with a transformer power supply. Quite apart from costing you up to three times more than one with a switched mode power supply, and having a limited operating voltage range, there are still heat issues associated with transformers.

I have witnessed a PFR with a hole melted in its housing from just this cause when crush mounted with a contactor. The primary benefit of using a transformer is galvanic isolation, which is not generally required in this type of device.

Choose a supplier whose PFRs are known to use top quality components, including self-repairing ‘3000 hour, 105oC’ capacitors that follow the ‘doubling 10oC rule’. In other words, at 105oC the rated life is 3000 hours. For every 10oC reduction in temperature the rated life doubles. Therefore at 65oC the rated life is 48,000 hours at 55oC, 96,000 hours, at 45oC, 192,000 hours and at 35oC 384,000 hours – or 44 years!

Malcolm Greenhill is with Charter Controls

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