Friday, August 7, 2015

By default, E3+ overload relays have a "reset" button that doubles as a "test trip" button

Long time, no blog!

At work the other day we came across an interesting situation. We had a motor that we just couldn't get started. Wiring was good, communication was good, no faults being recorded...but every time we hit the start button the motor instantly tripped.

Here's how we figured out the issue.

At Vanscoy, all of our big expansion project MCCs (motor control centres) use E3+ relays and DeviceNet for motor protection and control. These relays are like the circuit breakers in your house; they trip if there is too much current passing through them, but unlike your home's circuit breakers you can program them with more advanced and specific settings for different types of trips.

While in your house you switch a tripped breaker back to the ON position to reset it, these E3+ relays have a "TEST/RESET" button on the front. If your motor pulls too much current, the E3+ trips to protect the equipment, and an operator has to go hit the reset button to clear the fault.

(the faults are also re-settable over DeviceNet, so programmers can enable different behaviours like reset functionality with an HMI or a field stop button)

There is a programmable "Test Enable" parameter (#103) in the E3+ relays. When enabled, this parameter changes the changes the behaviour of the reset button on the front of the E3+ as follows:

  • An instantaneous TEST/RESET button press clears the faults/trips from the relay and allows you to resume operations. 
  • A button press >= 2 seconds generates a test trip, causing your motor (if running) to stop. 
A potash mill generates a lot of dust and humidity, and mechanical devices - including buttons - are failure-prone. What we had here was a sticky reset button that was maintaining a test trip. As soon as the motor was given a start command, it would trip. 

The solution was a two-parter:
  1. Fix the sticky reset button (we don't want faults to be instantaneously cleared without properly understanding them) 
  2. Disable the "Test Enable" parameter on the E3+ so that the "TEST/RESET" button could only clear faults, not generate them. 
Taking a step back, our problem was that we never bothered to understand what the "TEST" part of "TEST/RESET" meant - we had no idea this "Test Enable" feature existed, and didn't know the single button (that we thought was simply a reset button) had different functions. 

I think the "Test Enable" feature is a probably a good one. During construction and commissioning, being able to generate a test trip allows for testing the trip protection on the relay, as well as the functionality of any PLC monitoring the trips. 

But for anyone searching the 'net for E3+ relay info - I'd recommend that as part of a commissioning procedure, the "Test Enable" parameter be disabled as one of the final steps. This insulates plants from tricky-to-troubleshoot failures caused by sticky reset buttons. 

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