Strange, Possibly GFCI, Failure ??

Scenario:

Intermatic (motor-driven clock) timer fed from GFCI into 12VAC lawn-lighting transformer

When timer reaches turn-on point it appears that voltage drops enough that timer stops turning, so lights don't turn on.

Manually "pushing" timer thru turn-on point, lights come on just fine, with transformer "thumping" from surge, output voltage is correct.

Timer runs just fine on another outlet

Is it conceivable that GFCI is resistive unless loaded heavily?

How would you test?

...Jim Thompson

I don't think so. They are pretty much wire and contacts all the way through- and I don't think the sensing coil adds enough inductance to bother things with a single turn primary. Maybe its just very marginal and perhaps is on the other half of the 120. Have you measured the voltage at the outlets?

Our fancy Loran one got its timer fouled up (I think the #$(*#$ earwigs got into it) so I put an inexpensive Noma electronic timer ahead of it, and it's been working fine since, and switching some high voltage exterior lights at the same time. It uses a dirt-cheap 5A relay inside, so it will eventually fail, even at 2 operations per day, but that's no big deal.

Best regards, Spehro Pefhany

If I wanted to know if the GFCI was working at a low load, I would load it lightly, and measure the voltage. How lightly? What is the motor on the timer rated at? Probably 15W... use a night light as a load.

-Chuck

I read in alt.binaries.schematics.electronic that Jim Thompson wrote (in ) about 'Strange, Possibly GFCI, Failure ??', on Thu, 13 Oct 2005:

Maybe the contacts don't 'wet' on low load current.

Put a lamp in parallel with the transformer primary.

Jim, There is nothing between the input and output except a two pole relay contact. The torroids will not have enough inductance to affect regulation. There is currently a regulation test required by UL 943, the GFCI standard. You can probably get an idea of the relay contact condition and regulation by measuring the voltage at the face of the outlet with no load, and then with a heavy load such as a hair dryer or iron plugged in.

If you want to pull it out of the wall you could plug in and operate a hair dryer and measure the voltage at the line and load terminals of the GFCI. This would verify the GFCI and confirm the branch up to it.

I've never seen them have a high contact impedance with a light load.

Hope this helps.

Hello Jim,

That's odd. As Spehro said GFCI are pretty "digital" devices. Either the internal relay is on or off. If half on it would go bzzzzt.

Could it be that the timer is in a slightly different mechanical position between the two outlets? If it's one of those black hardware store editions with a lid over them they have plastic gears. Our experience so far: After a few years they make faint but weird grinding noises, after another year they work only when Mars and Venus are in perfect constellation, a few months later they are toastissimo.

Listen to the timer closely. If the gear noises are a very faint and regular hash it may be ok. If it sounds like a pulsating grind it's on the verge of seizing.

Regards, Joerg

Sure it is conceivable, but not likely. More likely that a wire feeding the system has a bad connection that is sucking all your voltage when you increase the current draw, such as would happen when the motor is pressured to turn the switch on. My experience with GFI's is they either work or they don't. Many times I find that people don't tighten the wires to them very well. But even then, they either work or they don't.

Jim,

First, use your DVM to confirm your suspicion that the voltage drops at the motor when the load goes up just before it trips on. If it doesn't, then the timer has a problem as others have suggested.

If it does, then backtrack to the GFCI and check the voltage from the output screw to neutral to see if it is there. If not, then you've got a loose wire at the output of the GFCI or somewhere downstream toward the timer.

If you see the voltage drop at the output screw of the GFCI, then assuming you are using an outlet GFCI, check from the input screw to neutral to see if the voltage drop is also at the input. If it is there too, then your problem is at the input connection to the GFCI or upstream toward the breaker panel. If you are using a breaker style GFCI or the voltage drop isn't at both sides of the outlet style GFCI, then the GFCI is likely at fault.

HTH

You hit it right- those little synchronous motors draw nothing and it is probably running off some quasi resistive and capacitive feedthrough due to a film layer on the GFCI contacts. The motor takes enough of a voltage drop to prevent voltage drivethrough of the GFCI contact but not enough to power it against the cam at timeout. You can test that theory by simply hanging a 25W load in parallel with the timer, and it should turn on with no problem, this should also work if you wait until the motor hangs and then plug a load in parallel with it, voltage should come back up and the timer should continue through its cycle without assist. You could try switching to a receptacle with incandescent pilot, or a new GFCI- I am assuming you're talking about a GFCI panel breaker and those are on the pricey side. Your other test with plugging into another receptacle is inconclusive unless you know whether that circuit was loaded or not.

Wait until it's stalled and connect a jumper (or two, as needed) across the GFCI. (If you don't like the taste of electric juice, wear rubber gloves.)

-- john

Jim, just for comparison, I have a LV landscape lighting system as well. It also uses an Intermatic (Motor-driven) timer/transformer pack. The pack is plugged into a GFCI outlet, via an nextesion cord, on a circuit dedicated to yard outlets.

The timer/transformer pack is the only utilization device on that brach circuit, and the timer operates just fine.

From this, it is possible that the timer may be defective, or that a bad connection exists at the GFCI contacts or screw terminals, as others have suggested.

Hope this helps.

Thomas