[garybdavis]

I got tired of getting on my knees to turn the on/off switch on the back of my plasma cutter so I built a relay box to turn it on. The plasma cutter sits under my water table along with the Bladerunner electronics. All the 120V stuff is wired to a single switch conveniently mounted on the front of the table. But the plasma cutter is a 240V/50A machine, so I had to turn it on and off by flipping the switch on the back of the machine. My knees and back where telling me it was time to get this project done.

The box switches a NEMA 14-50 plug on/off with a big, heavy relay that is activated by a 120V coil. So, I can plug the relay box into my 120V outlet strip that is turned on with a standard light switch. One flip of a switch and everything is powered up. I use 14-50 plugs instead of 6-50 throughout my shop so that's why this box is made for that. Plus, the 14-50 plug gives me access to 120V inside the box which allows me to include a "manual" switch so I can bypass the 120V remote input and turn it on with a flip of a switch. Not really necessary but kind of cool to have.

The printing is not silk screen but made from a 81/2x 11 sheet of clear label material. I created it in CorelDraw just like I do parts for cutting but send it to the printer. Since it's a scaled drawing, it prints out with true dimensions. I put "cross hairs" where the holes go so it serves as a drill template also.

The holes were made with Greenlee punches. I have the square one for the C13 connector for the 120V input. A standard computer power cable turns on the relay coil.

The indicators are 22mm LED from ebay. They come in 120V and 220V versions and multiple colors.

It was a fun project!

Gary

[john-100]

Hi Gary

I expect your relay box is some thing like this :-



John

[garybdavis]

Vert nice and almost exactly as I did it!

One difference is the ground. The ground coming in from the 14-50 plug is passed through to the 14-50 receptacle and bonded to the box and the ground from the C14 input is not connected to anything. This is to prevent a grounding loop as you only want one ground path back to the breaker box.

It may seem like nit-picking, but the plasma cutter can generate a lot of noise and you want it to have a dedicated ground all the way to the breaker panel. Also, if you are using a metal box like I did, don't let it come in contact with the plasma table as the box is bonded to ground. A plastic box would solve that problem but I hate using them so I put rubber feet on my box.

Gary

[john-100]

Hi Gary

I did have second thoughts about connecting both earths together and considered adding some high current diodes
to break the earth loop as sometimes used with audio equipment to avoid disconnecting an earth



living in the UK the domestic single supply is 240V between live & neutral
(with a 3 phase supply its 240 V between any phase and neutral / earth)

a lot simpler than your 115 / 230 V system

( historical decisions determine what we have to work with )


John

[garybdavis]

I would just simply not connect the C14 ground. It may seem counter-intuitive to not hook up a ground, but in this case, the box is fully protected using the 14-50 ground so including the C14 ground doesn't buy you anything.

[john-100]

Hi Gary

the C14 ground is only needed if the 230V supply was disconnected and you had a stray connection between the 115V control supply and the metal box

although If you have earth leakage / GFI protecting the supplies
I can see you will need to leave the C14 ground disconnected to stop unwanted interactions between the circuits

John

[garybdavis]

Good point about the control supply being connected with the 230V disconnected. I think that falls into the "double failure" category as you would have to have an electrical fault from the control signal while the box is not fully connected. Maybe add a warning to the label saying the power must be connected prior to connecting the control signal. I suppose neither grounding scenario is completely perfect.