is there such a thing as a "fake" voltage reading?

Joseph_4

Hello all,
I was at a call a few weeks ago where i was getting 27 volts at the boiler s8610u spark pack but we'd only get sparking. the gas valve wouldnt open up and there would be no voltage on pv and mv/pv terminals.. some electric wiring guru found there was a wire going from the c terminal to boiler jacket and therefore was "showing voltage " when voltage wasnt really present.

On Friday I was at a call where the boiler wouldn't fire. if i would measure the voltage while connected on gas valve at terminals th and tr on the spade prongs id get no voltage but if id REMOVE them from gas valve id get 25 volts. we thought bad gas valve so we changed, got same result.. we got this same reading while jumping out spill switch and flame roll out switch. we ran a wire straight from r and c terminal on transformer and gas valve fires.. This lead me to this fake voltage idea again.. How can the meter read a steady 25 volts and gas valve not fire?

Its quite bothersome cuz what else do we have "out in the field" except for a meter to see whats going on
thanks
Joe
HHI Services

Jamie Hall

Such voltage readings aren't "fake" - and are surprisingly common. However, to see how they get there one needs a little electrical theory. Let us suppose that you have a nice little electric circuit, with your transformer (say 24 volts), then a wire, then a load of some resistance -- call it A -- then your meter (with a resistance of around a megohm (sometimes a lot more than a megohm; some meters less), then a wire back to the transformer. What voltage will the meter read? Turns out it depends on A. If A is 0 -- a nice straight piece of wire -- it will read your source voltage; 24 volts. But what happens if it is more than 0, but much less than your meter? Your meter will still read pretty much the same as the source.

Now let us suppose that you take away the meter and put the gas valve in there -- or some other load -- with a resistance we will call B, and put the meter across the valve terminals. What do we read? Maybe something quite different. In fact, we can figure it out: The voltage across the load -- B -- will be equal to our source voltage times B, divided by A+B. Suppose A was 1000 ohms -- not unusual for an iffy connection, and B was 60 ohms -- high for a coil, but not wildly so. With no load in there, the meter reads 24 volt (actually, for a 1 megohm meter, 23.976 volts, but let's not get picky here). Put B in there, and the meter will read 24 times 60/1060, or about 1.35 volts.

What you need to do is measure the voltage across the gas valve (or whatever is bugging you) with the item connected. If it reads substantially less than it does with the item disconnected, you have a bad connection or other high resistance (could be, for instance, an unhappy transformer) and you need to find it.

And all you need is your meter and a good eye...

JUGHNE

That was a wise move to direct wire the gas valve for testing purposes to avoid changing the wrong part.

Any switch or device that develops high resistance before completely failing can fool you that way.

Had a 5 HP motor not starting, meter showed proper voltages on the top of the switch. Turn switch on and voltage disappears.
Back up to check fuses---good readings top and bottom. Bottom readings disappear trying to start motor. It turns out that blown fuse was not completely blown, you might say it was hanging on by a thread. The "thread" would power your meter but not your motor.
Sometimes the old method of using a 200 watt light bulb to check for power is necessary.
A simple wall switch can almost fail in the same manner.
Some older/cheaper meters that draws more power as it measures voltage might load the "thread" up enough to where the voltage falls on it's nose.

But that is 120 volt issues.
For 24 VAC, I wonder if the coil of a new working 24 volt contactor would confirm that the module switching was working or not....I have never tried it....but will if the chance arises.

DZoro

I like to cut out and replace all the old spade connectors in that circuit in those situations. Molex's are a pain, but must be cleaned and confirm good contact.

Harold

Depending on your meter, be sure you have set it for AC; not DC from the transformer. If set wrong it will affect the displayed voltage. Sorry to make such a simple point; not trying to annoy. Stuff happens.

ratio

Another thing to watch for is unloaded SCR or triac style controls. Without a load connected (contactor, etc.) they will show full voltage regardless of the state of the output.

Joseph_4

Thanks for the comments. well appreciated!~

Solid_Fuel_Man

My meter has a lowZ function where it lowers its impedance to put more than a megohm of load on a circuit. Helpful for induced voltage.

I've used a pair of automotive light bulbs as a load when troubleshooting 24v circuits. Those incandescent bulbs dont care about AC vs DC. I've found a good many high resistance devices/connections that fooled my meter into seeing full voltage but when a load was applied the voltage dropped or went away altogether.

Joseph_4

how do i set it up with the bulbs?

ratio

Hook them up as a 'load', that is hot to common, in such a way that they'd light up if there was power. With your meter, probe both before and after connecting the lamp. If you read voltage before and not after, either it's begin switched by an SCR/triac (electronic 'relay') or there's a high impedance connection somewhere.

You can actually do a whole lot of diagnostics with the lamp alone. I've been toying with the idea of making something like the old automotive tester lamps. In fact, that sounds like a fun project for the next few days!

JUGHNE

Ratio, are you using 2 -12 volt bulbs in series for the 24 vac testing?

ChrisJ

https://www.amazon.com/OEMTOOLS-25886-6-24V-Circuit-Tester/dp/B0014WIVX6?th=1&psc=1

@ratio

ratio

@JUGHNE, nah, my Fluke has a Low Z/auto ranging setting that I use, I haven't had any issue with it. 'Course, I haven't had to chase any high-impedance faults with it yet either. I've used lightbulbs in the past though, as a real electrician, both as a test light and as a current limiter to locate shorts. In fact, somewhere around here I've got a cord with an inline lamp holder & test probes on it. (Shhhhh! Don't tell OSHA!)

@ChrisJ, that's what I was thinking about, but making my own little insert to replace the lamp with a few LEDs, & maybe beep or something (& not blow up!) if I probe line voltage with it.

ChrisJ

> @ratio said:
> @JUGHNE, nah, my Fluke has a Low Z/auto ranging setting that I use, I haven't had any issue with it. 'Course, I haven't had to chase any high-impedance faults with it yet either. I've used lightbulbs in the past though, as a real electrician, both as a test light and as a current limiter to locate shorts. In fact, somewhere around here I've got a cord with an inline lamp holder & test probes on it. (Shhhhh! Don't tell OSHA!)
>
> @ChrisJ, that's what I was thinking about, but making my own little insert to replace the lamp with a few LEDs, & maybe beep or something (& not blow up!) if I probe line voltage with it.

If you want to actually load the circuit you're going to need some resistors across those LEDs.

ratio

Yup. It's never as easy as it sounds to do it right, but I'm going to have some fun learning how!

JUGHNE

In an old grocery store I came across the owners test light.
It was made up for him by an electrician.
It consisted of 2 WP pigtail/sockets, ( I use them for temp lighting before the actual fixture is installed), they were wired in series with a 60 watt bulb in each. If the 2 outer leads were applied to 240 the bulbs would glow normal. If testing 120 then less light.
This kept the owner from an exploding bulb as he was never sure what he was measuring....also self testing that the bulbs both worked.

If you could find auto bulb sockets with 2 leads showing then the same method would test for 24 volts/AC/DC and load the circuit to avoid the "fake" phantom voltages.

Another point is that my DVM will auto range and show MV, if I do not look closely at the range I am looking at some wacky readings from a non functioning circuit.

EBEBRATT-Ed

I have that problem with my Fluke auto ranging. Have to look closely at the #s. And my eyesight ain't what it used to be. Good meter but it drives me nuts.You think you have power and it's just a little phamtom voltage

JUGHNE

For back up meters I have a couple of older Amprobe AM-20's.
No auto ranging. You get what you see. So light that if dropped it is not a crash and burn.
A lot of features in a basic economical meter. Does 80% of what needs done.

ChrisJ

I did some changes to my boiler the other night and I've got two meters. A Fluke 179 and a Simpson 260 7P. For boiler work I chose the Simpson no question.

JUGHNE

I have the Simpson 260-7. Great to see the movement of a needle that you won't catch sometimes with a DVM. Great for bench work….a little big to haul around for basic TS though.
IIRC, a set of Simpson test leads cost about what the little AM-20 did years ago.

ChrisJ

> @JUGHNE said:
> I have the Simpson 260-7. Great to see the movement of a needle that you won't catch sometimes with a DVM. Great for bench work….a little big to haul around for basic TS though.
> IIRC, a set of Simpson test leads cost about what the little AM-20 did years ago.

My Fluke 179 goes for a little over $300. I can't stand cheap meters. That said, I still don't care for the Fluke for certain jobs.

Really, if I had the money I'd buy a portable scope.

ratio

I too prefer expensive meters to cheap meters, especially since I'm trusting my life to them so often. Since I'm sure I'm missing something by not having a good analog meter, I just ebay'd one. A late Christmas present I guess.

ChrisJ

> @ratio said:
> I too prefer expensive meters to cheap meters, especially since I'm trusting my life to them so often. Since I'm sure I'm missing something by not having a good analog meter, I just ebay'd one. A late Christmas present I guess.

Simpson 260 my friend. Can't go wrong.

I recommend a "P" one as this means it has a circuit breaker in series with the fuses.

A 7 doesn't have the breaker but a 7P does. I think the 8 series is the latest.

ratio

And so starts my collection.

I can quit any time. I can, I tell you!

Solid_Fuel_Man

I have a cheap Greenlee clamp meter I bought at a big-box many years ago. It's small and has never let me down. I carry it in my box which gets all the miles put on it. I have a Fluke 87 and a Klein Tough-Meter CL800 which I really like.

I've always carried a "hot stick" or non-contact voltage tester in my shirt pocket. I can honestly say I have NEVER been shocked on the job, and it's a rare day that goes by which I dont work with live voltage of 120-480.

ChrisJ

> @Solid_Fuel_Man said:
> I have a cheap Greenlee clamp meter I bought at a big-box many years ago. It's small and has never let me down. I carry it in my box which gets all the miles put on it. I have a Fluke 87 and a Klein Tough-Meter CL800 which I really like.
>
> I've always carried a "hot stick" or non-contact voltage tester in my shirt pocket. I can honestly say I have NEVER been shocked on the job, and it's a rare day that goes by which I dont work with live voltage of 120-480.
>
>

What do you do for a living?

Solid_Fuel_Man

@ChrisJ
I'm an industrial control electrician as my signature says. I wire many boilers and integrate building management systems.

Also install heat pumps, and light commercial boilers. And wood, pellet, and coal boilers.

Jamie Hall

That hot stick has to be one of the most useful gadgets ever. I always carry one, too. Comes in handy as a tiny screwdriver, too -- and just slips in the pocket along with all the other junk...

ratio

You can depend on a hot stick for two things. It'll either light up, or it won't.

I actually carry the Amprobe one, it lights up, beeps, & vibrates. And no on/off switch. Been pretty reliable the last decade or so. I'll be bummed when I have to replace it.

ChrisJ

Solid_Fuel_Man said:

@ChrisJ

I'm an industrial control electrician as my signature says. I wire many boilers and integrate building management systems.



Also install heat pumps, and light commercial boilers. And wood, pellet, and coal boilers.

"Master electrician specialising in boiler and burner controls, multiple fuel systems, radiant system controls, building controls, and universal refrigeration tech."

Hmm.

ratio

BTW, my (first of many?) Simpson meter arrived today. If I ever divorce Fluke I think I'll try and hook up with Simpson.