I was asked to trouble-shoot a garbage disposal that would not run when the switch was turned on, but worked fine when plugged into the receptacle for the dishwasher.
Obviously a bad switch, I thought, but a quick test with a non-contact tester showed that the switch was working.
My non-contact tester showed voltage at the GD receptacle as well, so I plugged in an outlet tester to see if there was an open neutral or something, but it showed “open hot” (no lights)
I pulled out the Fluke volt meter and measured 29V. Went back to the switch and measured 29V on the hot terminal. I then opened the service panel and found 45 year-old pushmatics and, sure enough, the breaker output for the circuit in question was 30V. All the others tested 119V. Cycling the breaker restored it to 119V, but I advised the HO to call a licensed electrician to replace the panel with something new and reliable.
Problem solved, but I’m puzzled. How can a faulty breaker suddenly drop output voltage to 30V? The indicator said it was “on”, but with those old Bulldogs the indicators cannot be trusted, so might it have tripped partially? But how could it just reduce voltage instead of opening the circuit?
Replies
It's possible the breaker has arced and left carbon on the contact points, this build up can reduce current flow. Cycling the breaker can get you better contact.
Pushmatic breakers can suffer from a lose screw where they attach to the buss bar, turn the main off, check the buss bar for voltage before tightening the screw. If one screw is lose check the others.
I gather that your Fluke was a digital voltmeter and what you meausred was a "fluke".
A digital voltmeter has very high resistance.
The breaker was open and what you measured was "phantom voltage" or capacitive coupling to the other hot wires in the box and in the wall.
William the Geezer, the sequel to Billy the Kid - Shoe
"I gather that your Fluke was a digital voltmeter and what you meausred was a "fluke"."Isn't the unofficial company slogan "If it works, it's a Fluke."? Yes, I use a Fluke T5-600 digital volt/amp/ohm meter.I put one probe on the output lug of the affected breaker (which is one pole of a dual breaker on a three-wire circuit) and the other on the neutral bus and read 30V. Similarly testing the other pole of that dual breaker gave me 119V.Are you saying that the breaker was actually open and that my meter was picking up induced voltage in the red wire from the companion black wire in that three-wire cable?So, even though my non-contact tester (with which I first tested the wall switch) is rated for 90-1000 Volts, it also sees phantom voltage as low as 30V?BruceT
Actually, it's capacitive pickup, not "induced". Exactly the same as the non-contact tester, which is why it shows a voltage.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Yes, that the breaker was open. And, in this case the most likely source of the phantom voltage was probably the other leg in the 3 wire cable.But you can get similar readings with almost and open wire running without a load on it."So, even though my non-contact tester (with which I first tested the wall switch) is rated for 90-1000 Volts, it also sees phantom voltage as low as 30V?"Those are not 100% reliable. And with the meter on it it might have loaded it down enough that that it might not have indicated..
William the Geezer, the sequel to Billy the Kid - Shoe
Bruce,
I'm partial to Fluke products myself...if you want the right tester for everyday electrical work, the Fluke T+ Pro is just the ticket:
http://us.fluke.com/usen/Products/Fluke+TPRO+T.htm
Cliff
Phantom voltage. With a high-impedance voltmeter you'll measure capacitive pickup from nearby wiring. The unconnected wire is acting as an "antenna" of sorts. Perfectly normal and probably not an indication of a problem with the panel/breaker. There may be other reasons for replacing the panel, but a popped breaker from an overloaded GD isn't one of them.
Since I measured 30V at the breaker post, I did not consider phantom voltage as the cause, but now that I think of it, the breaker is a dual 15A on a three-wire circuit, so that could be it.Part of the reason I recommended a new panel is that Pushmatics have a bad reputation for reliability and I had been called to check out a dead circuit two weeks before at the same house where the offending breaker showed "on". It took several tries cycling the button to get it to reset. Similarly, the "30V" breaker had to be cycled on/off several times before working. Given the bad rep for Pushmatics and two such failures in the "on" position, I have doubts that those breakers would protect in case of a real overload, so I recommended that the HO have the panel checked out by a licensed electrician.BruceT
I advised the HO to call a licensed electrician to replace the panel
This is not meant to criticize, but to educate along with the previous technical explanations of others here.
You do realize you gave 'lawyer safe' but otherwise poor advise? Did you test any of the breakers before you gave the 'expensive' fear-mongering advice.?
After reading the correct explanations of capacitively coupled phantom voltages earlier in the thread, are you going to contact the homeowner and report the findings (due to your misinterpretation of the readings?) that folks here have advised you of??
FWIW, My mom's house has 55 YO pushmatics (Sears version, even helped Pop put some in when I was 10 YO) and all are fine, checked 'em out** 6 years ago (plus everything esle in the house) when Pop died - aint about to replace them with homeline cr@p or some such from a big box.
--- Although, the 1915 main 100 A main knife switch fuse box could use cleaning up, which is sceheduled for this fall.....
** checkout procedure - 50A breaker on spec grade plug, plug into circuits, turn on 50A breaker, make sure the panel breaker trips. Paper clip thru a pencil eraser works also and can be more 'fun' - an eighth grader taught me that when I was in fifth <G>
PS: worthwhile to check the connection tightness on all the breakers and to figure out WHY it tripped - was the HO chopping up T-bones with the GD? Other outlets on the circuit?
Edited 8/9/2009 5:57 pm ET by junkhound
Yeah, I will follow up with the HO, but I'll still advise them to get the panel checked out by a licensed electrician. Since two breakers have tripped in the "on" position and had to be cycled several times to reset, I don't feel qualified to assure them that the rest are ok even if I were to test them.As to your confidence in pushmatics, you may be interested in this building inspection site that discusses them. http://www.inspect-ny.com/electric/Pushmatic.htmPushmatics are apparently not as bad as I was told by a couple of electricians I've talked to but, due to the lack of magnetic trip mechanism, not up to modern standards either.BruceT
Thanks for the link.
Even after reading the link, I'm still very comfortable with the breakers in Mom's house. The circuit breakers used in commercial airliners are all just straight thermal breakers.
Exercised all her breakers 6 years ago, will do so again this October (Mom is 2000 miles away).
If anyone reads the link part about 'sticking', do not spray with WD40 or such. Dont ask how I know that, let us just say having done some explosive atmosphere testing, you do not want to open any breaker under load that was recently sprayed with wd40 or similar <G>