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Electrical Question

It is totally irresponsible to state "this or that" voltage will not harm you. All voltages should be considered potentially lethal. While this may be considered an extreme example, a 1.5 volt "D" cell battery if applied across the human heart with needle points could be fatal. I always wear leather gloves when working around an AC power source and always use an OSHA approved fiberglass ladder in lieu of the cheaper aluminum variety.
 
I am sure there are going to be a few issues of what is 'practice' and 'regulation' in the various parts of the world - but in the UK the following should be observed:

1. Anything (anything) connected to the household mains supply should be either double insulated (CLASS II) or have a safety earth (CLASS I). If an item of equipment has a safety earth (CLASS I) - and a metal case (or anything that is metal come to that) it should be bonded to the safety earth. There are regulations on the maximum resistance that can exist between the point of contact and the socket outlet. This is for your safety and should always (always) be connected.

2. The safety earth should never have a 'large' current flowing in it from the incoming mains supply live or neutral. If the circuit has a modern form of protection - the protection device will specifically look for an imbalance in the live and neutral currents and trip the circuit should they exceed 30 mA (or so). A small leakage current is permitted to flow from the incoming mains supply to the safety earth.

3. Anything else should be considered not a safety earth - but (say) a signal earth / screen for audio/visual equipment. These need to be considered in the overall design of an installation to avoid hum loops (circulating currents in the screens) etc. arising from imbalances in the differing mains supplies which the various items of equipment are connected to. These may be disconnected if desired (known as ground lifting). You can do this safely as the safety earth is always connected to protect the human from a malfunctioning piece of equipment.

Have a healthy respect for the mains supply (be it 240V or 110V)...

If battery powered - identify the stored energy source. A 3.6V cell could discharge tens of Amps (or more) into a short circuit very rapidly. There are a few cases of where head-mounted torches have burst into flames (notably due to counterfeit LiON batteries). I have seen a small toy helicopter burst into flames in the air - spectacular - but dangerous! No one was injured - but the fumes that were produced...

That's my ten pence worth...

Dave
 
Agent Orange said:
It is totally irresponsible to state "this or that" voltage will not harm you. All voltages should be considered potentially lethal. While this may be considered an extreme example, a 1.5 volt "D" cell battery if applied across the human heart with needle points could be fatal. I always wear leather gloves when working around an AC power source and always use an OSHA approved fiberglass ladder in lieu of the cheaper aluminum variety.
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Nothing will harm you if you carefully avoid it.

I wouldn't wear leather gloves without proper insulating gloves underneath.
 
Agent Orange said:
I've never had a problem with leather gloves when dealing with household current.
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Can't say I've ever tried on household voltages, but the electric fence will punch right through them! Probably wouldn't work well even on 120VAC if they got damp.
 
glitch said:
Can't say I've ever tried on household voltages, but the electric fence will punch right through them! Probably wouldn't work well even on 120VAC if they got damp.
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Same with rubber gloves if they are wet or even damp. Common sense has to to kick in somewhere along the line.
 
Agent Orange said:
Same with rubber gloves if they are wet or even damp. Common sense has to to kick in somewhere along the line.
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Not the actual electrician's glove liners I've got -- you wear them under a leather glove shell, they work fine damp/wet as long as there's no pinholes. Wouldn't be much use if they stopped working when you got sweaty!
 
And if you're going to wear gloves, those are the gloves to wear. With the leather protectors.

Any other gloves are asking for trouble. But the bottom line is that you shouldn't be intentionally touching anything live with the gloves, either. They're only there in case you slip.
 
I'm only an armchair elechicken, but I'm pretty sure that neutral is supposed to be at equal potential to ground, not connected to it.
Best case scenario, you trip a GFI if you try that. Worst case, your ground lug is now at mains potential, and all the hazards/potential damage that carries.
 
It's a little more complicated. The ground lead is not supposed to carry current; its role is protection. Neutral (or in the case of North American power, the center tap on the distribution transformer (120-0-120) is usually bonded to a bonafide rod-in the-ground earth connection in the distribution panel.

It's a hard distinction for many because you have two conductors at the same potential--put an ohmmeter between them and you should see a low-resistance short. Current code says that "thou shalt not use the ground wire to carry current".

Way up the thread, note that I pointed out that this was not always the case, in my kitchen oven example. Three-wire 240V connections are common in homes built before about 1994--and I don't recall offhand any reports of electrocution by a permanently-installed electric kitchen oven. In fact, the connection made between the 1950s and 1994 was most commonly aluminum wire, which is another whole ball of wax.

When I had a new heat pump installed a couple of years ago, to replace the one installed in 1991, the electrician noted that the original was connected via grease-filled wire nuts from aluminum (from the breaker panel) to the copper of the heat pump. He said that he had the training and the hydraulic tool to crimp a copper pigtail to the aluminum, but decided to simply replace the wiring with new copper--it was only about a 10 foot run, after all--he said it was less expensive to do so.

The point is that NEC is not a static thing and changes over the years. Personally, I'm dumbfounded at some of the Chinese garbage that's plugged into our mains wiring. I
 
Yeah, I've got four-wire split-phase in my house as well (120-0-120), same as you see for any electric stove or dryers. Not sure what's supposed to happen after 1994.
Aluminum wire isn't a problem for split-phase so long as it's thick enough gauge to meet electrical code for the current you're trying to pass through it (to avoid overheating). Aluminum has been phased out completely for 14 gauge cable though (my house is old enough to have it in some of the runs).

I live in Canaderp, but I'd wager the US plays by similar standards.
 
As far as I know the NEC is pretty much the same on either side of the border.

As I understand it, stranded aluminum cable is not as much of a problem as the old solid-conductor stuff. I suspect that a fair number of service drops still use it.

In my shop, I do have a foot of the stuff the local utility uses for 12KV buried cable. It's insulated copper and very heavy. Must cost a fortune for one of those big spools you see being towed on trailers behind utility trucks, but probably not as expensive as 1200-pair telephone cable.
 
It's not a current carrying issue with aluminum, it's oxidization causing high resistance connections at splices, breakers, receptacles, etc. which leads to heating and then fire. Even when you run the heavy aluminum stuff for a service entrance into the breaker panel, the main lugs or breaker has to be rated for aluminum, and you have to use a "noalox" anti-oxidization paste for aluminum. That's the "grease" in the "grease-filled nuts" Chuck mentioned (many folks call them "purple twisters," I don't know if that's a trade name or what...some of them are in fact purple). Receptacles and everything else likewise must be rated for aluminum (or dual rated, you often see Cu/Al on them somewhere) and you've got to use the grease there, too. On big lugs, getting the right torque is more important with aluminum, too.
 
The visiting electrician did mention that it's a good idea to periodically give the terminals connecting to the aluminum wire in the breaker panel a little "torque" every few years.

I've also seen copper-clad aluminum wire, which I assume doesn't have quite the same problem.
 
NeXT said:
Calling a pro...

So right now I have a rather large UPS in one of my three racks and right now it's doing a very inefficient job powering one infrequently used HP 9000 system instead of the rest of my servers where I really need it due to frequent drop-outs. The UPS has an L14-30P (X, Y, neutral and ground) plug for connecting to the breaker panel but in the output side I only have an L6-30R (X, Y, ground) receptacle to connect a PDU. For the HP that's fine because I can flick it to 230v service and that's fine but the second rack has several 120v-only devices, so I have to use a PDU that has both 230v (x, y) and split-phase 120v (X or Y, neutral) but that then comes with an L14-30P, which won't connect to the UPS currently.
Replacing the receptacle on the UPS is fairly trivial as it's just a regular box mount unit....

41Zum-1dK8L_grande.jpg
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...however adding the Neutral raises questions.
If I remember right, in Canada and most of residential North America, the Neutral wire from the wall, at the breaker panel and at the pole is just ground, however it is only allowed to connect directly to ground at the breaker panel and at the pole. Doing so anywhere else isn't allowed.
My theory is that so long as my neutral to the breaker panel isn't broken, I can route the neutral from the UPS input to the output and in a power failure the UPS will handle the 120/230v and the neutral will still work on its way back to ground and everything will still operate. Is this correct?
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No it won't. You household 120v is really 120-0-120 split phase. It looks like the output from the UPS is a true European style 0-220v not a split phase. So neutral from the breaker should still be at zero referenced to the UPS output. To produce a split phase output would require much more circuitry if you allowed un-balanced loads on the outputs.
 
The 120v outlets on your stove (if yours has them) and electric stove-top elements, if you're in North America, all run on 120v, going between 120 and 0 (neutral), while the oven elements run between the two 120v lines. Because household power is "split phase," each 120v rail at the service panel is at opposite phase (180 degrees) to the other, giving you 240v if you connect between them. European and other countries household service usually runs at 220-240v single phase.

If you're in a large office building, or industrial complex, there will be an electrical room with multi-tap transformers (probably including a high-leg delta) that take three phase power and drop it to single or split-phase 120, 208v, 240v, 277v, rails, etc to go to the service panels.

Also forgot to mention last time that there are circumstances where the neutral line may not be at zero-potential to ground, especially if a three-phase system has an unbalanced load.

But back to the topic, What model is the UPS?

If the UPS has a split-phase input, one would think that you'd just add the output neutral connected to the input neutral and be done with it, but this might leave your 120v devices floating in the event of power failure, among other electrical no-noes, because when a UPS trips into battery mode, the AC input is disconnected, but you'd still have your bodged neutral connected. You'd also run the risk of an unbalanced load at 240, producing current ripples if the UPS isn't designed for it. It may be that that particular model is actually providing split-phase output, but only outfitted with the 240v output plug (120-120) sans neutral, and in that case, all you need to do is change the plug. But I'd think it more likely, given how these things monitor voltage and fine-tune the output, that it is only capable of single-phase 240v out.

If you want to be professional about it, I'd just replace the UPS with one that can do 120/240 out, or get another one for your 120v equipment.
 
blackepyon said:
The 120v outlets on your stove (if yours has them) and electric stove-top elements, if you're in North America, all run on 120v, going between 120 and 0 (neutral), while the oven elements run between the two 120v lines. Because household power is "split phase," each 120v rail at the service panel is at opposite phase (180 degrees) to the other, giving you 240v if you connect between them. European and other countries household service usually runs at 220-240v single phase.
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But I was talking about a wall oven, which uses 240V elements. AFAIK, only the clock runs from 120VAC.


If you're in a large office building, or industrial complex, there will be an electrical room with multi-tap transformers (probably including a high-leg delta) that take three phase power and drop it to single or split-phase 120, 208v, 240v, 277v, rails, etc to go to the service panels.

Also forgot to mention last time that there are circumstances where the neutral line may not be at zero-potential to ground, especially if a three-phase system has an unbalanced load.
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Depends on the configuration. A system with a wye secondary usually has the common center-tap grounded/neutral, where all phases measure 120V to netural, and 208V phase-to-phase. A delta-secondary transformer has no neutral--all you get is the phase-to-phase voltage of 240V, unless it's one of the hybrids with a single center tapped phase. The center tap of that phase then becomes "neutral" and the setup supplies 120-0-120.

There remain only a few place in the US using the two-phase distribution system (4 wires with each pair out of phase by 90 degrees), but they're still around.

But back to the topic, What model is the UPS?

If the UPS has a split-phase input, one would think that you'd just add the output neutral connected to the input neutral and be done with it, but this might leave your 120v devices floating in the event of power failure, among other electrical no-noes, because when a UPS trips into battery mode, the AC input is disconnected, but you'd still have your bodged neutral connected. You'd also run the risk of an unbalanced load at 240, producing current ripples if the UPS isn't designed for it. It may be that that particular model is actually providing split-phase output, but only outfitted with the 240v output plug (120-120) sans neutral, and in that case, all you need to do is change the plug. But I'd think it more likely, given how these things monitor voltage and fine-tune the output, that it is only capable of single-phase 240v out.

If you want to be professional about it, I'd just replace the UPS with one that can do 120/240 out, or get another one for your 120v equipment.
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Looking at what specs I could find on the UPS, there is only a single-phase output, with no taps. It's a purpose-built rackmount unit for the HP1000 system, IIRC.
 
Chuck(G) said:
It's a little more complicated. The ground lead is not supposed to carry current; its role is protection. Neutral (or in the case of North American power, the center tap on the distribution transformer (120-0-120) is usually bonded to a bonafide rod-in the-ground earth connection in the distribution panel.

It's a hard distinction for many because you have two conductors at the same potential--put an ohmmeter between them and you should see a low-resistance short. Current code says that "thou shalt not use the ground wire to carry current".
...
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Not exactly. The code says to not use the grounding conductor as a current-carrying conductor. The terminology chosen is specific and precise, much like computer terminology. The grounded conductor (again, NOT the grounding conductor) is typically a neutral (whether the center tap for a single-phase or the common point for a three-phase wye or the center tap of one side of a three-phase delta is irrelevant) but the code term is grounded conductor, and it is by definition the current-carrying conductor that is bonded to the grounding conductor at one point, which is in the service disconnecting means in normal services. The phase 'distribution panel' is very ambiguous and not used by the code for that reason. This is one area where I have significant and long-term training.

The point is that NEC is not a static thing and changes over the years. Personally, I'm dumbfounded at some of the Chinese garbage that's plugged into our mains wiring. I
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The NEC is updated on a schedule, every three years. We're due an update in 2020; my last NFPA 70 book is the 2017 edition.

On the subject of gloves, well, NFPA 70E has all those requirements. I have a full HRC2 arc flash suit myself (and I am trained in its use), including a fresh set of gloves purchased this past year.. I have to suit up just to do some of the measurements I need to do with our Fluke 435 three-phase meter, which I have to connect while the equipment is hot, where the equipment in question is a Mitsubishi 9900B 500KVA UPS with an 800A 480V delta feeder (and 160 12V 540AH batteries in four 540VDC cabinets for a 30-minute 100% load run time) that is contractually required to stay up 24/7/52. This is something I know a little bit about.

The statements about about voltage are very correct and very true; typically the danger point is considered to be 50VDC and above. But respect all voltage, treating every voltage source as being capable of harm, and you'll be ok. Some voltage sources (such as the 12,470V medium voltage feeders here) are more 'aggressive' in their ability to harm you than others, but nearly any voltage can cause harm if it's in the wrong place (watch the 'Baghdad Battery' section of the 'Cooling a Sixpack' episode of Mythbusters for a demonstration!).
 
lowen said:
Not exactly. The code says to not use the grounding conductor as a current-carrying conductor. The terminology chosen is specific and precise, much like computer terminology. The grounded conductor (again, NOT the grounding conductor) is typically a neutral (whether the center tap for a single-phase or the common point for a three-phase wye or the center tap of one side of a three-phase delta is irrelevant) but the code term is grounded conductor, and it is by definition the current-carrying conductor that is bonded to the grounding conductor at one point, which is in the service disconnecting means in normal services. The phase 'distribution panel' is very ambiguous and not used by the code for that reason. This is one area where I have significant and long-term training.
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I think we're splitting hairs here. Don't use the earthed lead to carry current.
 
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