240V power supply wiring diagram

[rsdoran]

Looking at the print, it still confuses me what they are connecting to. I see the number 2 (X2, neutral) but I also see the ground sysmbol. Is the print saying it's connected to both the ground (metal backplate, metal inclosure) and X2?

As jimbo mentioned earlier the Pilot Light is a push to test type, the secondary contacts should indicate that. An example PPT light:
http://web6.automationdirect.com/static/specs/eaton30mmptti.pdf

It has 2 sets of contacts, NC set connected to ground, NO to neutral(2 or X2). In its normal state (power on>lite ON) it indicates power is on AND NO ground fault condition has occured. Light is OFF and Push to Test and light comes ON indicates a ground fault condition.

Never thought this part of the circuit would be questioned so intensely. At least this thread took a positive direction.

 

[DonsDaMan]

I think the problem comes about because no one draws their schematics like anyone else. I know there are standards, but standards have to be interpreted, and interpretations may vary.

I saw my first Air Conditioning schematic about 10 years after I was introduced to electricity/electronics. I couldn't make heads nor tails out of it, so I pulled apart the panel and started crossing wire colors with color abbreviations, and then the boxes with components... after about an hour, I finally could read the schematic.

Similar to the circuit Ron posted, the circles being connections is "standard" to the guy who drew it, but when I've done similar schematics, I've called out both the NC and NO contacts on the drawing to show the connections.

My way isn't "right" and the way shown isn't "wrong." But the two of them are different from one-another.

Anyway... that's my 2 cents' worth.

 

[darrenj]

If it does abide by NEC standards maybe it's because the wiring is inclosed in a control cabinet.

Looking at the print, it still confuses me what they are connecting to. I see the number 2 (X2, neutral) but I also see the ground sysmbol. Is the print saying it's connected to both the ground (metal backplate, metal inclosure) and X2?

Let me try my best to explain what both Bruce and I are trying to say.

Let me use a home with a un-attached garage as an example. The NEC says a seperately derived system is to be bonded (neutral to ground) either at the transformer or the main disconnect switch. In my home the main is in the circuit panel so this is where my system is bonded. Meaning there is a green screw that goes through the neutral bar and screws right into the metal panel.

So now I want to run power out to my garage. I want to put a 60 amp panel out there. So I run 2 hots, a ground and a neutral out there. I put my panel in and I screw another green screw through the neutral bar to the metal panel. Oops, NEC told me to ONLY bond my system at either the transformer or the main serice disconnect. So now my big Oops just created what some call parallel paths. The current can now flow through the neutral AND the grounding conductor. Say now someone is out digging one day and almost hits the feeders but just hits the neutral and chops it in half. Or you just loose the neutral for one reason or another. You don't it's been cut, lights out in the garage still works.. everything must be fine. Guess what.. now ALL the current is returning through the grounding conductor or metal pipes back to the service. Still no problem until the poor sucker that takes the conduit appart that feeds the 3-way switch between the garage and the house. As the homeowner, I would start praying that you didn't kill the poor guy.

Ron, this may be a poor example but it is a problem if you ground a system in more than 1 place.

Please, no offense Ron..

Gary

WRONG!!!

Sorry now that i have your attention!!
You sound like you know the code to some respects so i will assume you are a sparky?

o.k. If you are you will understand this right away if your not it might take more explination...

Any Class 1 transformers..(class one being a type of transformer that will sit there and burn if shorted) Must be bonded to ground on the secondary side.Yes the code says bond only in one place..however read it carefully and you will see it says per system (Or something to that effect) When you use a transformer it becomes a different system. This applies to small 50va xformers all the up...Rons circuit is used all the time. The code states that it MUST be used if you are not grounding the secondary side of the transformer..

Why bond the secondary side??..Try this one out the next time you wire a transformer....

wire the transformer as usuall but do not ground the Neautal side..Run the hot thru a small fuse say 2 amp the ground the wire after the fuse..Notice i said ground..Dont short to neautral...What happens????NOTHING..why? becouse the transformer isnt "Referenced" to anything..

As for using rons circuit we put them in all the time in older buildings that have "Delta" services (No neutral)

Example..
600v 3phase "delta" service incoming..(For the US people that gives you approx 347v to ground) Now if one line should ground itself out..Short in a motor or bad wiring..nothing happens to the system,no breakers trip, this is becouse the ground isnt "referenced"
Diagram below!!

Phase A------------------------Light bulb A-----ground
Phase B------------------------Light bulb b-----ground
Phase C------------------------Light bulb c-----ground

O.K. so we have say phase C grounding..what happens?? Light bulb c will go out(Think about it for a min) and light bulb A and B will go from dim (Remember a 600v light bulb buring at 347v) to full bright becouse now you now have 600 across it...

 

[darrenj]

also respectfully ron that part of the circuit would be redundant becouse if the system got grounded the fuse would blow..If however the system was ungrounded it would be a major Plus for the guy troubleshooting..

 

[rsdoran]

You are correct darren, it should blow.

I think I missed some points on the ground aspect. As an FYI in the US you are required to ground the transformer and one conductor on 120 volt 2 wire or 240/120 3 wire systems.

I just love the NEC Handbook, it has pictures.
Bruce GMc if you get a chance take a look at a copy of NFPA 79.

 

[93lt1]

Bruce GMc if you get a chance take a look at a copy of NFPA 79.

I think I remember reading something about the "ground OK" light in NFPA 79. I have certainly seen several of them installed. I have even seen some company's "general equipment specs" require them for each of the key ground points within the panel(s). It seems that most of the machinery I have seen them on was metalforming machinery, like this "idea" might have come out of the auto industry (just guessing).

Earlier, someone mentioned the "push to test" type arrangement. All of the ones I have seen installed, the push to test connected the light to nuetral, the normal state was light to ground.


All of this picky NEC criticism, How about this:

There's a good chance the breaker or fuses feeding this panel are 20A or greater. I've seen this type of arrangement tapping into a 200A supply with 14awg.

This is a good example where the NEC does not apply to industrial contol panels.

 

[darrenj]

Ron i totally agree but here in canada if it runs from a transformer it has to be grounded no matter what the voltage..from 2 volts up..(perfect one to leave for the inspectors!! First question they ask me when they see a control transformer..."Did you bond the secondary side??"...I keep a straight face (Its getting hard) and reply" Damn i forgot about that..." I then take out a short piece of green wire and do it infront of them...Once they have found an item they are happy and go off to the next call...In there mind it justifies the 100odd bucks you have to spend..


The "Delta" systems i am talking about are a blast from the past...hydro will no longer allow you to install these systems...however downtown there is still quite a few of them. Untill you work on it then you have to upgrade it (That is if hydro has run the neautral wire down the poles). The "delta" system was quite popular in industy becouse if a motor or circuit grounded out you could still finish production then work on it rather than haveing the line down and fixing in a rush..Something to be said for both ways..

 

[darrenj]

All of this picky NEC criticism, How about this:

There's a good chance the breaker or fuses feeding this panel are 20A or greater. I've seen this type of arrangement tapping into a 200A supply with 14awg.

This is a good example where the NEC does not apply to industrial contol panels.

But the NEC states that the wire tapping off the 200amp supply must be no more than x feet (In canada it 3 feet) and must be enclosed in the same enclosure or protected with steel pipe.. another way to look at this is your incoming main service on your house..There is a fuse on the transformer at the road..Lets say the transformer will deliver 1000a (Common for residental hood's becouse 1 transformer feeds way ore than one house..) From the transformer to your house the wire (In my case a #3 rateds at 100amps is actually fused at 1000 amps)
This is o.k. becouse the fuse protects the wire..If i was to draw 150 amps in my house the main breaker in my house would trip before the wire melted..If there was a short before my main breaker (the only way for that wire to draw anything above 100amps) the fuse on the transformer would blow protecting the wire... V over I times R wins every time..

 

[Lancie1]

Darrenj,

The Canadian electric utility is doing everybody a favor by outlawing "delta" ungrounded secondary power systems. It is true that a ground on one phase of a 3-phase delta system will not cause any breakers to trip, and everything can keep running.

There are two serious problems with this. What happens when another phase also faults to ground? Then instead of a phase-to-ground fault, you have essentailly a phase-to-phase fault, which is much more serious and usually something burns or blows up. The second problem is that once you have a grounded phase, you have to somehow locate it. This is not easy to do in a large plant. It involves a lot of circuit isolating and testing. I did some work at a plant with a secondary delta system. The maintenace foreman warned me that Phase C was faulted to ground and had been that way for over 2 years. He said that during every holiday, vacation, and plant shutdown, his crew worked frantically trying to locate the fault, but so far had no luck.

Compare both these problems with a grounded-Y (or "star" as you Europeans say). When you get a ground fault on a hard-grounded system, the closest breaker or fuse trips, the equipment on that circuit stops, you can go right to it and fix the problem. The circuit breaker does all the work of locating the fault for you!

I say it is time that severe restrictions were put on new installations of ungrounded delta systems in the US also.

 

[darrenj]

i do agree lancie1 ..as i i said it has it pros and cons. The US still allows you to put them in?

 

[jimbo3123]

Darrenj,

I say it is time that severe restrictions were put on new installations of ungrounded delta systems in the US also.

I agree. I would extend this to ungrounded single phase control power installations as well. Until very recently, it was the common practice at one of the major american automotive OEMs to use ungrounded systems for all control power.

I still see installed ungrounded systems on a regular basis. It just makes me shake my head and wonder.

 

[Lancie1]

Darrenj,

Yes, they are still legal here, and I know of several plants that have this type. About 3 years ago, I was designing a small remote facility at a TVA (Tennessee Valley Authority) fossil power plant. I specified a delta-wye pole-mounted transformer to supply the 480 volt power. My logic was that there would be no place in this small 8' x 8' building to put any type of grounded-phase detectors, so the grounded Y would be safer for the plant maintenance personnel. One of the TVA engineers questioned this and told me that they normally used delta-delta transformers because they caused fewer power outages. I told him that I considered this to be a safety hazard and if he insisted on using the delta-delta transformer, then to consider me off the project and he could finish it himself.

I can understand that electrical utilities like to use the ungrounded delta to apply electric current to overhead power lines, because a tree limb across 1 phase will not cause a power outage, and only highly trained linemen with the proper equipment will be working on the lines. These reasons do not usually apply in any other type of facility, and the not-hard-grounded delta becomes a hazard, in my opinion.

 

[rsdoran]

All of this picky NEC criticism, How about this:

The drawing shows the 14AWG enclosed in metal conduit which conforms to NEC guidelines. This is a common method especially where bus systems are used.

NEC/NFPA is applicable to any electrical applications in the US. OSHA states both NEC and NFPA 79 as the standard(s) to use. Insurance inspectors also use NEC/NFPA 79 as their reference.

I worked in the amusement industry for years. On a weekly basis I had to deal with inspectors...state (usually Dept of Agriculture which for some reason also govern rail roads), state electrical, county electrical, fire marshalls (which also did electrical), health, and insurance. I had to maintain a 5 wire 3 phase system with 3-8 foot grounds (triangulated) per gen set. We had to do Xrays, magnaflux, cable tension tests, regularly calibrate torque wrenches, ground fault monitoring, probably one of the first industries required to use GFCI. The insurance inspectors could be the most demanding. In Atlanta GA I even had an electrical inspector wearing bib overalls and barefoot inspect after a rain by touching things.....this REALLY HAPPENED but the real issue was a political battle that we got caught in the middle of.

I learned that if you follow or exceed OSHA/NEC/NFPA standards that in most cases it will alleviate any issues that may arise.

NOTE: If you dont believe any of this then go work for a carnival for a week or so and see for yourself. I just stated a few of the requirements we had to meet.

 

[brucechase]

OK guys, last time for me. I did go out and get a 2002 NFPA79 and actually saw the picture that Ron posted early on. I read through the entire manual (as much as I could - a little dry reading) and even though there is a picture, I could find no reference to the circuit.
My point on this (that seems to have gotten lost a few times) was that I've never seen this and I wanted to know more about it. Depending on where the light is grounded, if the ground is lost (and the light goes out except when you push it) and a person is touching the panel, then they could complete the circuit through their body (NOTE: I said could depending on where the light is grounded). NFPA 79 section 8.1.2 says to me that a grounded circuit (the control circuit) should have only 1 ground not a second (the light). AGAIN, I am trying to learn and understand this. I understand the function of the light and a PTT button along with ungrounded systems, corner grounded system and grounded systems. It was also interesting that section 19 requires testing of the ground by specific methods that does not include a light. I figured that it would be in this section as a ground verification.

Last item I found Ron was that section 9.1.2.1 doesn't allow control voltages over 120 VAC. Very interesting reading though, thanks for pointing out that I need to look at more than just 1 book for all the details.

I hope no one takes this post as derogatory, I am learning like many others here (even though I've done this for 17 years).

BGC

 

[rsdoran]

Depending on where the light is grounded, if the ground is lost (and the light goes out except when you push it) and a person is touching the panel, then they could complete the circuit through their body (NOTE: I said could depending on where the light is grounded).

The PPT light has 2 sets of contacts, a wire connected to ground goes to the NC set, if you lose the ground ...ie wire breaks etc then the light loses its reference. The panel is/should ALWAYS be at ground potential and the light fixture is mounted on the panel so the fixture is at ground potential. The only way I can see a person completing the circuit to the PTT is if they hold the panel, which should be at ground potential, then touch the NC contact of the PPT.

I am not sure what you mean by "could depending on where the light is grounded". Whether its a wire going to the ground bus or bolted to the panel I cant see a problem, the connection for the "light" is on a contact block.

I have been doing this stuff for many years myself and I have never been goood at explaining so everything I do "online" is to learn how to explain in better details. This example was taken from the 2000 edition of NFPA 79 pages 42, 43 & 44.