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Tech Forum

November 2016

GFI Tester

I have a GFI type breaker for my garage door that trips occasionally, making it so the door won’t open with the remote. I’d like to know how to test the GFI to see if it has gotten weak or failed in some manner.

What’s the best method to test it?

Henry Doran
Phoenix, AZ


I have to ask myself why anyone would put a GFI on the garage door opening circuit/electronics? I see no need for such a thing.

The reason we have GFI protected outlets is because someone could unintentionally become a ground path for the AC current. This could be deadly.

With a garage door circuit, no one should be playing with the circuit unless the electricity to the unit is off. It is a servicing issue NOT a use issue.

In other words, if the circuit is being used as intended, then there is no way you will inadvertently become a grounding path for the circuit. The circuit is protected in a housing and not available, as is an AC outlet, for you to come in contact with.

Disclaimer: This is an opinion of the sender only, not advice as to what you should do.

Phil Karras
Mt Airy, MD

You can buy them. You can make them. What is the normal trip level, you’d have to look up. A resistor (suggest metal oxide) from ground to hot, at about 6 mA should trip the GFCI. e.g. R<=120/0.006 The problem is more complex when you look here: I actually suspect that you may have an AFCI/GFCI combination and it’s tripping on an arc fault because of the motor. If that’s the case, I would consider an RFI filter (possibly medical grade which has a lower leakage) and appropriately sized ZNR’s. There have been reports of just plain flakey GFCI’s. So contact the manufacturer too.

Ron Dozier
Wilmington, DE

Working with live power lines can be dangerous. I purchased a GFCI tester from Home Depot similar to this one: To verify that the GFCI outlet can deliver higher currents, plug in a hair dryer or room heater. Make sure you don't overload your circuit. The trip level for a UL 943 Class A GFCI is 4 to 6 mA, or about 24K ohms from hot to safety. I don't recommend building your own circuit. Buy a device that is already built for this purpose.

Dennis Page
Beverly Hills, MI

As indicated in , a GFI is supposed to trip if there is 5 mA of difference in current flow between the hot and neutral (return) wires in the protected circuit. The "test" button puts a resistor between the hot and ground wires. You could put a larger value resistor between the hot and ground to see if the GFI does not trip with some smaller current. A 56K resistor, for instance would provide about 2.1 mA. If that trips the GFI, then the GFI may be too sensitive. It is more likely that there is an intermittent short to ground in the door opener, or that the door opener is too close to the current limit of the GFI (which also acts as a circuit breaker, usually 15 or 20A).

Phil Martel
Salem, NH

Your best bet is to replace the breaker — don’t waste the time, etc. trying to “test”it.

Ken Simmons
Auburn, MI

Many hardware or big box stores carry testers for receptacle testing, they have three lights to indicate correct wiring. Some of these have a test button to check operation of the of GFI devices.

Dave Bassett
The Villages, FL