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EHELP: We need a relay that can withstand high DC current at high voltages

EHELP: We need a relay that can withstand high DC current at high voltages

Q: My Model 330R process monitor with display has configurable threshold output relays that I need to set in order to activate a 125Vdc battery powered circuit which is part of a failsafe control room application. How would I set these up when standard instrument relays are rated at 125/240Vac and 24Vac/dc?

A: The Model 330R process monitor threshold alarm relays are ‘compact’ relays designed to activate ‘external intermediate’ relays. High DC voltage (and/or current) switching is always performed on the output contacts of a stand-alone external intermediate relay because high voltage DC switching (opening of relay contacts) creates a sustained high temperature plasma arc. This arc cannot be readily extinguished in a small instrument package therefore the relay sacrificially absorbs the excess energy. This ultimately requires replacement of the relay however this is a more economical alternative to replacing a blown panel meter.

A proven solution for the high DC current at high voltages need is to use Form ‘X’ style relays with a built-in magnet (this is a readily available industrial relay product which also comes in a DPDT configuration) as your intermediate relays. The coils of these intermediate relays are driven by standard voltage/current relays as in the Model 330R series. A quick Internet search of ‘Form X relay schematic’ will provide you with wiring instructions.

A Form X relay has an optional magnet (recommended for DC voltage applications). As the relay opens, a small to moderate sized electrical plasma arc always occurs. The magnet pushes the arc away from a straight line and onto a longer distance pathway. Thus the Form X relay’s mechanical doubling of contact separation distance along with the properties of the installed magnet significantly increase the arc travel distance and more quickly extinguish the DC plasma arc. When wiring for DC switching, be mindful of the polarity of the electrical contact pathway through the magnetic field to ensure that you are extending the arc distance.

To optimally select a Form X relay:

  • Perform an Internet search for ‘Form X relay with magnetic blowout’
  • Specify a clear relay package referred to as an ‘ice cube’. This permits visual inspection of the relay’s internal movements.
  • Specify the relay ice cube package with either a DIN rail socket or a panel mount socket with a retaining clip so the relay does not fall out of the socket as may occur in high vibration applications.

To learn more about the Model 330R, download the datasheet.

Published: May 23, 2016