Film-Tech Forum ARCHIVE: Electrical Safety question

This is topic Electrical Safety question in forum Film-Yak at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=8;t=005482

Posted by Richard Hamilton (Member # 321) on 06-02-2008, 09:08 AM:

This weekend my parents air conditioning in their house quit. I looked at it Saturday morning and found that it had a bad transformer which feeds the control circuit. The bad transformer had a Line input of 240VAC with the load being 24VAC. I could not find a replacement transformer with an input of 240VAC. What I bought was a transformer with an input of 120VAC.

My question/concern is this: The air conditioning unit has 220V and a ground coming into it. I hooked the new transformer up with one side of the input to one leg of the 220V and the other side going to ground (since there is no neutral). Are there any safety issues because I am using a ground instead of neutral?

I don't want to go outside and lean against the AC unit and end up looking like Yahoo Serious!

Thanks, Rick

Posted by Mark Gulbrandsen (Member # 72) on 06-02-2008, 09:15 AM:

Not really. The secondary is obviously isolated fomr the primary and as asuch the control circuitry wouldn't be in danger. I suggest however that you get the correct tranny fomr the manufacturer or at least one at Grainger as the ooriginol may have been thermally protected in such a way as to prevent a fire.

Mark.

Posted by Richard Hamilton (Member # 321) on 06-02-2008, 09:38 AM:

thanks Mark, I will tell the AC company to bring that part before they come to do their PM call on it soon. I was just concerned about the safety of the electrical. It was one of those " I can make it work, but its not right" fixes, and I wasn't sure about the electrical. And I didn't want to call a service company on the weekend to replace a $17 part.

Rick

Posted by Mark Gulbrandsen (Member # 72) on 06-02-2008, 12:37 PM:

I agree! ou may want to just get the part yourself and save some on that too. The HVAC company will probably double or triple their cost on it to you....

Mark

Posted by Bruce Hansen (Member # 281) on 06-02-2008, 05:28 PM:

There should not be current flow on a ground line. Any time there is current flow on a wire, there will be a voltage drop on that wire (E=I*R). Your A/C casing would have a voltage on it. Now here is reality: the current that xformer is going to draw is fairly low, and therefore the voltage drop would also be very low. This should work until you can get the correct part.

Posted by Charles Greenlee (Member # 3856) on 06-02-2008, 05:33 PM:

quote: Bruce Hansen
E=I*R

Reminds me of high school physics every time I see Ohm's law. Or teacher sang it to the tune of "Twinkle Twinkle little star" It was sooo stupid, I actually remember it. "Twinkle Twinkle little star, E equals I times R."

Posted by Louis Bornwasser (Member # 3063) on 06-02-2008, 06:32 PM:

Most popcorn machines and stoves are wired "that way." The neutral is also the ground. New rules require 4 wire (2 hot, ground, and neutral)but there are PLENTY (including my house) that are wired the old, 3 wire way. Works out OK as long as the ground/neutral is oversized. Louis

Posted by Steve Guttag (Member # 268) on 06-02-2008, 08:00 PM:

Ground and Neutral are certainly not the same and what you did is certainly not safe and not to code.

While Neutral and Ground do go to the same place (Earth), what Bruce stated is absolutely correct. There is no way to conduct electricity without causing a potential difference and thus raise the "ground" to a potential above ground. It compromises the entire grounding of the building and the safety of the wiring as the ground is not allowed to have a potential.

The code has traditionally been...the first load-center (breaker panel) after a step down transformer may connect the Neutral and Ground busses together as they are one and the same at that point and ONLY that point. This is because that is where the building's ground rod is tied to the panel...the potential at the busses will be identical. Any sub-panels after that, including any branch circuits MUST have a separate ground. Since there will be current flow on the Neutral (unless the currents are prefectly balanced), the Neutral and ground will be at different potentals all all circuits.

Can you get away with it electrically in a pinch.....sure, you did...is it safe...not really, depending on the wiring.

Steve

Posted by Richard Hamilton (Member # 321) on 06-02-2008, 08:36 PM:

Thanks for the replies. I was curious about the safety issue, it will be replaced this week. I was just in a "pinch" and wanted my parents to have air conditioning this last weekend.

The ground wire/neutral that I hooked up from the transformer is physically screwed into the sheet metal of the AC unit, the 120AC line I'm getting is from one side of the contactor. The old transformer got the 220VAC from the input to the contactor.

Posted by Stephen Furley (Member # 1277) on 06-03-2008, 02:00 AM:

Richard,

You said:

quote: Richard Hamilton
I hooked the new transformer up with one side of the input to one leg of the 220V and the other side going to ground (since there is no neutral).

Where is this? Your location is given as China, but getting 120 ish Volts by connecting between one leg and ground sounds like the US or Canada, with a centre tapped supply, yet you also mention 220V, which isn't a standard Voltage there. There are major differences between electrical systems, and regulations in different places. I can't see any way that you could connect a 120V transformer in this way in China.

Posted by Richard Hamilton (Member # 321) on 06-03-2008, 06:01 AM:

Stephen, sorry about that! I am staying at my parents in the States now, will be back in China probably next month.

Rick

Posted by Fred Tucker (Member # 4482) on 06-10-2008, 03:47 AM:

THe reason you didnt have a neutral on old stoves, dryers and the like is there were no electronics or accessories that required 120V. With the advent of such things as LED clocks on the stove, the code changed to allow for this safely. Was twsisting wires long before I fell in love with film.

Posted by Stephen Furley (Member # 1277) on 06-10-2008, 12:50 PM:

quote: Fred Tucker
the code changed to allow for this safely.

I doubt that there's actually any safety issue here, though it may be required by regulations which claim to be on grounds of safety.

In the roughly half of he world which has a single phase Voltage of around 240V dryers, cooks etc have lamps, moters, electro-mechanical timers etc. operating on 240V without large numbers of people being killed by them. At one time there was probably an issue of being able to use standard, readily available, parts, but these days the same factory, probably in China, is likely to be making components for use in many different countries, with different Voltages. Also, these days things like timers are likely to be electronic devices operating on a few Volts d.c., and this could be derived just as well from 240V as from 120V, so there's probably no great advantage in having a neutral available these days. There is an advantage to 120V for small incandescent lamps are concerned, and 120V ones are certainly more readily available in the US than 240V ones, but I doubt that it's really a safety issue. You could always use a 12V halogen lamp.

Posted by Louis Bornwasser (Member # 3063) on 06-10-2008, 02:52 PM:

In fact, USA stoves have had 120v devices since at least the 1950's. The fluorescent light, oven light, and clock motor on my Mom's old 1959 stove are all 120v. The distinction is that the ground and the neutral cannot be the same now. Back then, they were the same wire....and no one died because the wire was large enough to hold, blowing the fuse in the "hot" wire.

Conversely, the load return for 120v is now carried by the neutral; the ground is purely safety being tied from the frame to earth. Louis