wiring a heat treatment oven

E

elementfe

Joined
May 3, 2008
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All ready to wire up my oven, and I realized one gigantic (to me, anyway) thing is missing.
Following this diagram, what don't you see?
http://www.bladeforums.com/forums/s...eat-Treat-Oven-Wiring-Schematic-Future-Sticky

Everything.....but the ground.

I have to confess, even after wiring my home and shop by the book, I'm not sure I understand the function of the third wire in a three wire 220v circuit. A dryer or range, for instance, will have a dedicated chassis ground as a fourth (green) wire, but oh no, not welders and such....

The two hots are 220v apart on a sine curve, I get that- the potential difference is 220v.
But is the third wire comparable to "Earth" in a 120v system? Is its function solely to be more efficient at grounding a short than your body is, or does it function partly as a "return?"

How have you (anyone that wishes to respond) dealt with that issue in home built appliances?

Thanks for any insight!
 
In a 220 VAC system, the voltage comes from a center tapped transformer. The center tap is referenced to earth/ground. Reading across the ends gives 220V, and reading to either end from the center would give 110V. The two end taps are called L1 and L2 , and are black and red ( occasionally just black) in circuits

The white neutral is that "earth" tap. It is in many 220 devices, because not all the components run on 220. Many things, like lights fans, and timers only need 110V.

The green "ground" is a cabinet/chassis ground. It makes the cabinet at the same potential as the neutral wire. If there is a fault between the hot leads and the cabinet, it trips the fuse/breaker. If the cabinet was not tied at the same potential as the neutral line the cabinet could have 110V "floating" above earth. Any person or object that was grounded to earth and came in contact would create a path to ground. This can range from annoying to deadly.
The separate green wire run from the power box is a way to assure that the "earth" grounding is there, and not rely on the white neutral to do double duty.

So, in the diagram shown, the schematic is for the oven coil and control wiring. It is only for showing how the electronics and resistive load are powered. There will be other devices like fans that probably will need 110V. Proper wiring would also have a chassis ground, and a neutral line ( to power the 110v items ). The 110V items are usually powered by a connection between L1 and neutral. They should have their own switch and fuse, too.

Floating around on BF somewhere is a more complete wiring diagram, which shows the 110V parts and the ground.
 
What Stacy said above is good advice. If I might comment a bit more - in the schematic/wiring diagram you linked to they have everything (even PID) running on 220vac. Nothing wrong with that, and many electronic things do - they actually run on a MUCH lower voltage and have an internal voltage regulator that drops the line voltage down to required voltage, and this voltage regulator doesn't care if it's 120VAC or 220VAC it has to regulate. This allows functioning in other countries that use 220VAC as normal voltage as well as USA where the "normal" voltage is 120VAC.

As Stacy said, there are many 220VAC power items that require both 120 as well as 220, and these will normally have 4 wires, L1, L2, a white neutral, and a green chassis ground wire. For things like a welder which only as 220/240vac components (even the fan in my welder is 220), then 3 wires work just fine, L1, L2, and green chassis ground.

In the linked wiring diagram, just add a green wire and connect it to chassis ground and you have a complete "correct" wiring diagram.

My wife says she has all the electrical knowledge required: "that stuff will bite you!" {g}

Ken H>
 
Thanks for the replies! Very helpful.
I think I'd best find a complete diagram and make absolutely sure I understand it before going any farther- I agree with Ken's wife that being very very afraid of that stuff is a good policy.
I have both versions available to plug into (4- and 3-wire 220), so it would be best to choose one. My control box is wire mesh on all but the face, so doesn't have a fan or any other 110 needed, air cooled, which makes me lean toward a simpler 3-wire system.

What I'm working toward is a definitive yes/no answer to the question: "Should I treat the third wire (usually white, right?) as a chassis ground or Earth, and ground the frame of the oven to it?"
 
Well Sir - there is no truly "safe" definitive yes/no answer. "IF" it is a 3 wire circuit, then the 3rd wire "should" be green for chassis ground. "IF" it is a 4 wire system, the it should be two colors (red/black, black/black), with a white wire and a green wire.

Now, will your PID controller run on 220vac? You have no 120 vac items, (fan, lights, etc)? In the linked wiring diagram it shows a "power LED" which is not clear. It "should" say something like "220 VAC power light" to indicator the light is rated for 220VAC. Also, I'd question the location of the fuse, and only having one fuse in a 220vac just isn't good. It should have two fuses, one for each leg.

I don't see a need for the "power LED" because anytime there is power present, the PID is going to light up and be it's own "power indicator". No need for another power indicator.

"IF" your PID is 220vac rated, and there are no other 120 vac requirements, then the linked wiring diagram is just fine, and do use a 3 wire system, and the 3rd wire "should" be green for chassis ground. I might think of having 2 fuses, one in each leg. This fuse is a 1amp or so because the only power being used is by the PID and it's going to pull fractions of an amp. The 220vac power to the heating elements are protected by the breaker on the circuit and by the main fuses before DPST switch. I just read the whole thread and see someone says the 3rd wire is white, as in neutral. The neutral should NOT be connected to chassis ground. The white neutral and green chassis ground wire should be kept separate from each other until reaching the final point at last panel/connection point in the system, just before the main ground is connected to the ground rod.

If you take a look at your electrical panel you will see the neutral bar (white wires) is insulated from the panel itself, while the ground bar (bare copper) is connected to the metal panel. This keeps the neutral and ground separate. This separation allows a GFI breaker to function by measuring any current flow (should be none) between neutral and ground. If the GFI detects a small (mA) of current, it trips providing personnel protection.

Do you have a 220 vac recept you plan to use that's already wired in? OR will you have to install a recept from breaker panel?

Unless your PID is exactly the same as the PID in wiring diagram, don't use the terminal numbers on PID or SSR, but check your PID/SSR for terminal numbers to use.

I just noticed the "Heat LED", Another thing - this circuit is getting signal power from PID to power the SSR - this should be a low voltage, but again, not knowing what PID you have, it's hard to say - also depends on how your PID is setup, for modulating control or simple OFF/ON control.

Anyway, I've rambled on here - any questions, Stacy, myself, or others will be happy to help. PTDoc is pretty good also.

Ken H>
 
I was pretty much thinking I'd leave the lights out of the system, for the reason you state.

The controller is designed to run on 220.

I see two fuses on the way "in," one on each leg by the main power switch. and another on the way into the PID.

The most convenient place to plug it in would be the receptacle I used to have my big welder on, a 3 wire. The third wire is white- I guess I can open the hatch on the welder or get on Google and try to understand just how the white us wired inside the unit.

For some reason my brain is going "clunk" when I hear that the white wire is, and is not, a chassis ground. When I wired the panel, I seem to remember the white going to ground both at the service entrance and the (fully grounded to earth) shop sub panel...but that was more than ten years ago.


If there's a green wire, which is obviously earth, then what the heck do you do with the white???
 
Check the panel wiring that goes to the recept for the welder. Is the white wire is connected to the neutral bar (insulated from metal cabinet and has all the other white wires).

What color are the wires going from panel to recept?

What color are the wires in the welder cord from recept to welder? If there is a white wire, see if it's used as a line wire or as ground/neutral.

Later - got to run now
 
While it is possible to wire a kiln and PID on a 220V three wire circuit, it isn't the normal method. I would go as far as saying that a neutral line is a requirement, and thus it would need a four wire circuit.

Your open mesh enclosure is good.......but.....make sure the heat sinks are good size on the SSRs if you have no fan. The PID has no heat sink...so it is running at ambient. The heat generated around a kiln can get close to both of these devices operating range. A fan is a simple and inexpensive device to help protect your PID and SSRs.

As far as lights go, I feel it is an important safety as well as operational feature for there to be a clear indicator that the power is on, the PID is on, and when the coil is being energized.
A green light just after the main power switch, a yellow light in the line after the PID ON switch , and a red light at one of the SSR 220V coil connections. Each light goes to neutral, and thus would be a 110V lamp. A glance at these three lights ( they should be side by side) will tell you the status of your kiln.
A door interlock switch that shuts off the DC power to the SSRs is also an important feature ( this switch is in the wire from the PID to the SSRs.).
 
Well, if you want power indicators, and a fan is nice to have on the SSR. Using the following would allow use of 220 VAC only and would not need any 120 vac. I was surprised to find 220vac LED lights so easy.

220 vac Green LED - http://www.ebay.com/itm/371151577179
220 vac Red LED - http://www.ebay.com/itm/1414189929413
220 vac fan - http://www.ebay.com/itm/321333363918

One problem with the requirement of 4 wires, is finding a 220VAC recept and plug. Stacy, you mention most kilns are 4 wire - I'm not at all familiar with kiln wiring, only with the Evenheat oven I've got which is 3 wire. Even Paragon electric ovens use the same NEMA 6-50P plug which is 3 wire. By using the 3 items above, there is no place to connect the white neutral wire, so why use it? I am certainly open to suggestions why the neutral is needed.

Stacy, please don't take my opinions as "confrontational" - I've the utmost respect for your knowledge 'n experience and the work you do as moderator. Being a moderator is often a thankless job, but you handle it great.

Ken H>
 
Thanks, Gentlemen, lots of good pointers in there!
FWIW, I pulled the covers on the appliances that I use on the receptacle I intended to use for the furnace (2 wire feed Millers) and
found that the third wire is directly grounded to the chassis, no ifs ands or buts. Only further question is, is the feed for that circuit tied into green or white in the panel, or does it matter. Since the neutral bus goes to ground in the shop panel, it's hard to see how it could matter.
The plan at this point is to hardwire into the panel in a different location with a 4 wire system and ground with green, have white available as a 110v return.
 
Ken, Thanks, I didn't go into how to make it all 220, as if there is 220, there is already 110. In some cases, like welders, there are no sub-circuits and everything can be run on 220 with only a ground.

elementfe -
In the USA, most ground busses are the same buss as the neutral...as it should be. The separate green and white wires are to prevent the possibility of a floating ground. A breaker would trip it there was a sufficiently low resistance short between the floating ground and hot...but a person might be electrocuted and the breaker would never trip. A GFI would detect the difference in potential even if only a few milliamps. Additionally,
the GFI would detect a differential between the ground and neutral if one was broken/disconnected. In some devices, shortcuts are taken, but for a device with a metal cabinet that has electronic controls and heating coils, I would consider the white and green wire both as necessary.
 
Thanks again for the clarification.
It's surprising in retrospect how much I didn't understand when I wired the place- just followed the manuals and trusted the inspection.
It's one thing to follow directions and quite another sometimes to understand why they are what they are...same thing happened with heat treatment! Sure, I made some good blades at first, but actually learning why it's the way it is.... it's like the bear went over the mountain- just when I think I'm getting a handle on this metallurgy thing, there's another mountain :)
May it always be that way!
 
Ken H> said:
From the PDF file above I think you can see why I mentioned the neutral buss and ground buss MUST be kept separate until at service entrance where the main panel is normally located.

Ken H>
Click to expand...

...unless you're in a seperate, metal clad workshop, in which case the panel is treated pretty much like a S.E., with double rod ground, neutral/ground bonded etc., correct?
 
Well, it all depends if your separate metal clad workshop has it's own service entrance with main panel. "IF" the workshop panel is a sub-panel from the main panel that is located at the service entrance, the ground wire MUST (by NEC code) kept separate from the neutral wires. Even with a ground rod (or 2 or 3 or 4) at workshop, those ground rods MUST (by code) be tied back to the ground at service entrance. All ground rods MUST (by code) be tied together to form a single ground system.

It's explained pretty clearly in the linked PDF file.

Ken H>
 
I've got a panel on the pole, by the meter, about a hundred yards from the house, with two rods.
A panel where the wires come back up and into the house, with two ground rods, and hot/hot/neutral from there to the shop, where there's another panel with two rods. That's what the inspector wanted. Hope it's right....
That means it's bonded to ground in three places, IIRC.
 
Let's see if I've got this in my mind correctly. You've got 3 panels, one main panel at a pole with ground rod there. This panel feeds the house panel, and the shop panel? Or - does this pole panel feed the house panel, then the house panel feed the shop panel? Doesn't really matter, these pole panel is still the "main" panel, and both of the other panels are sub-panels.

Are the ground rods connected together? From what I'm reading, you say "two hots and a neutral", that means there is no bare (or green) wire between the two panels?

Let's see, is this pole panel got several breakers? Or is it just a pole mount feeding the house main panel? Do I understand correctly, from this pole panel, there is only 2 hots and a neutral going to the house panel? and - from house panel to shop panel there is two hots 'n and neutral? No ground wire (green or bare) from either pole to house panel, or from house panel to shop panel?

You mentioned this is what the inspector wants? well, that settles it - remember, the NEC is NOT "law of the land", it's just a recommendation which most places and insurance groups have accepted. How long ago did the inspector accept this? The rule of tying all ground rods together is a fairly recent........ well, maybe that isn't true - I think it was back in the 90's and that's actually a LONG time ago. But to an old man, it doesn't seem "that" long ago.

Ken H>
 
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