Somewhat dumb basic electrical question

[gravitar 3]

I guess the question would be even more "dumb" if I didn't ask it, so.. Do you ground one leg of the secondary of your control transformer to the panel? If so, why? Do you ground one leg of the output side of your DC power supply to the panel? If so, why? I've seen both of these practices more often then not, and the benefit of doing so escapes me. In the other area in which I dabble with electronics, arcade video games, you have a 1:1 transformer strictly to isolate the secondary from ground. The reasoning is that you're safer because even if your body provides a path to ground, there's still no (or very little) current flow. The monitor chassis is often "hot" so being isolated is a good thing! Seems to me that you would have an analogous situation in a control panel. So why do people bond one leg of the secondary so often?

[Chris Elston 79]

Yes most certainly. Why? So you don't get yourself killed. If you did not ground the secondary then you would have a "floating" neutral state. Your transformer doesn't know which leg to make the neutral or ground, which causes a floating neutral. If this happens, what you thing is suppose to be the neutral wire could actually become the hot wire depending on which mood the transformer is in that day and if your holding a floating neutral at the same time your touching mother earth... As for the power supply, no I do not to this as a practice. The power supplies I get are low voltage 24VDC and they already have a seperate ground place to terminate.

[GerryM 2]

I don't know much about video games, but it is usually always safer to ground the secondary on the transformers especially with industrial equipment. The whole reason being to quickly blow a fuse if a fault occurs, preferably before a human becomes the fuse if you know what I mean. It is also required by code to do so. A video game is probably a special application, similar to plug-in hand tools. Remember when there were no cordless drills? In that application it was more dangerous to have a ground, due to inductance in the cord. So all hand tools are double insulated and have only a hot and neutral. Some hospitals have isolated systems also for various reasons. (lots of oxygen and patients with monitoring equipment on) Once again these are all special cases. Concerning the 24V DC power supplies, I usually do not ground them based on the way that I use them. They are not required by code to be grounded. However, once when starting up a new panel I had a 120V wire touch a 5V wire. Had the 5V supply been grounded a fuse wold have blown. Since it wasn't and the 5V supply was isolated from ground nothing happened until I came along and touched the 5V wire. I then grounded the 120V line and got quite a surprise. So from that point of view its not a bad idea to ground the DC power supplies as well unless you want things isolated. Hope that helps answer your question.

[Pierre 1]

Vlotage is a potential difference. By itself it does not exist! You will have an amount of electrons that will flow between two different potentials. Just as a rock will roll down a hill. Between the top of the hill and the bottom, there is a difference of potential. As for grounding one side of a transformer it is done to establish what and where resides the potential difference. Instead of waiting for a loose connector to ground on of the leads by mistake, we establish exactly which conductor is common / neutral / ground / hearth / and so on. That way we stand with a clear vision of what is HOT and what is NOT. What we can touch and what we cannot. A neutral conductor that would become loose and touch the ground will be of no concern. Not potential difference with itslef. On the other hand, if a HOT conductor touches ground, KABOUM! You will soon find out there is a problem. This is safer because all the surrounding materials become safety barriers against you or me getting zapped by on of them floating potential that could be HOT or NOT. For the 24 Vdc, I never ground it. It has save me so much equipments. Specially when an electrician or technician uses his ohmmeter to test voltage and he connect to 24 V+ and 120 Vac. Edited 19 Jul 2005 by Pierre

[TConnolly 13]

Just three weeks ago a maintennace spark asked for some help with a furnace panel. I stuck a volt meter on a relay coil and ground, and I read 59 volt when I was expecting 120. So I glanced over at the transformer, and sure enough, it was not grounded. Checking the other side of the relay coil, the one that should have been neutral, also yielded 59 volts. This is what is meant by the tranformer secondary "floating." So how did this make troubleshooting a pain for the maintenance guy? That relay was off. He expected it be on and so he traced through the circuit to find the component that was not working. But since there was a potential on the neutral side there was also potential backfeeding through the coil and up the circuit. Even if the relay been on, he still would have read the same voltages with respect to ground. This had the maintenance guy confused, because everywhere he checked both forwards and backwards through the circuit there was voltage. Don't ask me why he thought 59V was OK, I have no idea except that he is a young kid about 8 months out of trade-tech. I had the guy first bond the transformer leg to ground, and then he found the bad switch on his own in only a couple of minutes.

[Pierre 1]

Darn digital meters and darn digital megohmmeters.