A simple build for much peace of mind.
I recently came across a new (to me anyway) electronics youtube channel presented by one Peter Oakes. I love collecting these channels, many great designers out there and lots of good information to absorb. I may do a post just on my favourite ones in future. Anyway, he is making both a power supply and dummy load so I ventured through his videos to glean lovely pearls of wisdom.
One of his videos was entirely on building a safety outlet for projects under test, and oddly it’s one thing I overlooked. I was using a modified metal power strip which incorporated a 15A breaker and a switch to turn it on and off. I always felt it lacking though. 15A is a lot of current and even my beefiest of projects are not designed to draw more than 3A. Some tighter short circuit protection was always on the to-do list.
Though many hobbyists prefer to work using an isolation transformer, Peter had the novel idea of using just bits one could get from the Home Depot electrical aisle to make an outlet with built in safety features. His design uses a GFCI outlet which will trip if there is a short from hot or neutral to ground (or uneven current across the hot and neutral), a switch to turn it on or off, and a fuse.
He does contrast this with using an isolation transformer in that the GFCI will not function downstream from one. I would like to see a detailed debate on the two methods to see which one is “better” from a safety perspective or even if the two could be combined.
Off to Home Despot
Having need to pick up a machine screw for something else anyway, I decided I’d pick up the necessary bits and build my own. Pretty simple construction anyone could do and provides a great safety margin for the hobbyist.
Construction is simple, using off-the-shelf home electrical components. I selected:
- A cast metal dual-gang box meant for surface mount installation (i.e. not the metal boxes that are meant to be encased in a wall)
- A PVC front plate with a switch actuator on one side and a door to access the outlet on the other side (meant for exterior installations)
- A standard GFCI outlet (I had this on hand)
- A standard light switch (a whole $0.99!)
- A strain relief for the power cable with a 3/4″ threaded connection
- 3/4″ Hole plugs
- A case mount cartridge fuse holder (I had this on hand)
- A selection of cartridge fuses, 3A, 4A (Also had these)
- A beefy AC line cable with moulded plug (salvaged)
Construction is simple, mount the parts in the box, wire it correctly and close it up &nash; blammo. As is always the case, things never go quite as planned. The stupid box I bought has five threaded holes for ingress/egress of cables and strain reliefs etc. Yet the stupid thing came with only TWO screw in hole plugs. WTF? It is entirely conceivable that only one hole in the box would be used, why were four plugs not included? Given that the plugs were made of soft cast aluminum, I think the makers of this box really cheaped out on me. Fortunately I bought hole plugs.
Which, as it turned out, DIDN’T FIT. The box was stated as 3/4″, the hole plugs were stated as 3/4″, yet I couldn’t get the damn things in the hole! I believe they were meant for the bare metal boxes used inside walls so I lose points for buying the wrong thing, but still. 3/4″ is 3/4″! I blame the imperial system. So that’s junk. I ended up taping up two of the holes on the bottom from the inside, plugging the back hole using one of the supplied plugs and using the other to mount the fuse holder.
That in itself was entertaining. I had to drill a 5/8″ hole in the screw plug to mount the fuse holder. Lacking in proper clamps and vices to hold it in place, it was an hour or so of grinding the hole out using my dremel. My step bit would have caught it and flung it across the room. What a pain. Fortunately, the soft metal of the plug was easy to work with and I got the fuse mounted with no trouble and just some moderate swearing.
This is an important bit and must be done properly and safely. Working with mains is bloody dangerous so do make sure you know what you are doing. You could cause a fire or kill yourself very easily.
Wiring was simple, I stripped the AC line cable carefully (do not nick the insulation on the conductors inside) leaving a generous amount of wire for inside the box and bolted it into the strain relief into one of the holes on the top of the box.
The ground (green) wire I screwed into the green screw on the box (extremely important, earths the entire box).
The hot (black) wire, I then crimped into a spade connector which I connected to one of the fuse holder terminals and covered with heatshrink (important, no shorts to case!). I did likewise to another length of same-gauge black wire to the other terminal of the fuse holder. I then screwed the other end into one of the terminals on the switch.
Using yet another length of black wire, I connected the other terminal of the switch to the HOT terminal of the GFCI outlet.
I used a green wire to connect the ground terminal of the GFCI to the ground terminal of the box (extremely important for the GFCI to function).
Lastly, I connected the neutral (white) wire to the appropriate terminal on the GFCI. I then screwed both the switch and the outlet into the box and screwed the cover on.
Lastly, I installed a 4A cartridge fuse into the holder and mounted the box to the shelf of my workbench. Done!
Important things to remember when wiring mains
There are codes and conventions for wiring mains electricity for a reason. Parts and practices are designed to prevent accidental shorts and meltings leading to fires and death. I must here state: I am in no way responsible if you kill yourself or burn your house down. It is your responsibility to do this correctly. If you have a buddy who is an electrician, ask him! Do your homework. Do not kill yourself.
Here are a few tips for mains wiring for safety:
- Never, ever work on anything when it’s live. Ever.
- Use 14 gauge wire for 15A standard installations.
- Use solid core for wiring that is inaccessible and will not move. Use stranded wire for external cabling that will move and is meant to be flexible.
- Copper is brittle and will break if flexed too much, this is why we have stranded wire.
- Never combine aluminum wire with copper wire, this will lead to galvanic corrosion and the wire will break eventually resulting in a live wire flapping in the breeze.
- Always ground everything. If it has a green ground terminal screw, run a wire from that screw to the ground terminal of the box. Do so in a “star” configuration, with every groundable device leading to that one ground screw in the box. This ensures there is a minimal resistance to ground and doesn’t create any ground loops which can cause interference.
- Do not overstrip wire, strip just enough to go around the screw (or to insert into the terminal) and no more.
- Wrap wire around the screw in a clockwise direction and seat the wire inside the two little metal tabs that are designed to hold the wire in place. Tighten the screw securely. Clockwise winding means that tightening the screw tightens the wire’s hold on the screw.
- Beware when screwing in stranded wire that no wire “hairs” are poking out, they can cause shorts and injury.
- Close up all boxes securely so that no holes for fingers, tools, debris or liquids exist.
- Mount the box securely to your bench or whatever so it doesn’t float about.
- Use a box/case that is meant for exterior (i.e. not inside a wall) use.
- A GFCI on it’s own is insufficient protection. It is ONLY designed to trip if an unequal amount of current returns via the neutral than comes out of the hot (i.e. a short to ground from either hot or neutral). It will NOT trip if you make a dead short between hot and neutral (using a conductor up to and including your own body). A fuse or breaker is required to stop excess current flow.
I now have a pretty cool looking safety outlet for my desk. The lovely red handle for toggling the power is a nice addition I think. I’ve already made good use of it and blown several fuses so it’s usefulness cannot be overstated. In selecting low value fuses (3-4A) I can have it blow well before the mains breaker would (at 15A) giving me a much tighter safety margin. Fortunately, I have not yet tripped the GFCI (and very glad of that).