Power at Work: What to watch out for when doing your own wiring

Sunday, November 2, 2008

When doing small electrical repairs, make sure you understand the principles and correct procedures. It could save your life

by RALPH WINFIELD

While we usually hire an experienced electrician to do major wiring projects on the farm, many of us tend to do small electrical repair jobs in buildings or on equipment to save time and money. But we must be careful. We must understand things like wire gauge, polarity, voltage, amperage and voltage drop, as well as conventions such as wirecolours.

What precipitated this article was a very recent personal experience. I acquired a used, older heavy-duty drill press from a farm sale. It was just what I wanted for drilling those larger holes in steel – and the price was right.

My first concern was that the switch box was loose. After I unplugged the drill so that I could safely remove the switch to tighten the box mount bolt, I realized that the switch was mounted upside down. Convention has it that for a vertical switch it should be ON when up. This one was OFF when up. No problem to correct that glitch.

Before plugging the drill back into the receptacle, I noted that the plug was loose on the end of the cord.

Since the cord was much longer than necessary, I reasoned that I could correct two faults in just a few minutes. Was I ever glad I removed that plug!

It had been "cross wired," thus the neutral rather than the hot line was being switched.
Here are some things to watch out for.

Receptacles. All newer 120-volt receptacles have three openings on the face. The "mouth-shaped" one, usually at the bottom, is the grounded one.

It receives the round prong of a three-pronged plug. Note also that the two "vertical eyes" are of different sizes.

The one on the left is larger. It receives the larger, neutral prong if only a two-prong, non-grounded plug is used.

By the process of elimination, you will have surmised that the smaller vertical "eye" opening should be the one for the power or hot wire connection.  

Remember I said that most grounded receptacles look like a face with two eyes at the top and the one mouth (ground) below? Well, every rule has exceptions. Some receptacles must be installed the other way up to meet the code.

In hospitals or other locations where single metal receptacle covers are used, inspectors may request (demand) this orientation so that, if a loose plate falls, it will be caught by the grounded, round prong. I just recently noted this receptacle orientation in a hospital waiting room.

If you have never installed or don't plan to install or replace any receptacles, you can test every receptacle you have if you want. A receptacle tester like the one shown (on page 44) and used by hydro inspectors is readily available from electrical wholesalers or other suppliers for under $10.

Plugs. Even if you do not install or replace receptacles, at some point you are likely to replace the plug on a piece of equipment, even if it is only the tea kettle. Plug replacement often delays the purchase of a new tea kettle for a long time, because the wires next to the plug fatigue off from numerous wire bends during routine use.

The rules are very simple. Connect the white neutral wire to the silver screw and the black power wire to the gold/brass coloured screw. If the green ground wire is present, it connects to the green-coloured screw.

The tea kettle and many other tool cords are only two-wire. They do not have the green ground wire. Many tools requiring only a two-prong plug are "polarity sensitive." Thus, again, the white wire goes to the larger prong and the black wire to the smaller prong.

Two-wire cords and plugs are still very common on portable grinders, saws and wet/dry vacuums found in many farm shops. When the plug and receptacle are wired properly, the black or power wire will be switched, not the neutral!

Ground Fault Current Interrupters (GFCI). As you know, GFCI units
must be installed for some electrical circuits in order to meet safety requirements of the Ontario Electrical Safety Code. Usually, these are circuits in wet locations.

GFCIs compare the outgoing current to the returning current on the neutral. If they are not equal, it opens the circuit, thus the term interrupter, not breaker or fuse. The receptacle tester shown here will also test 120 volt GFCIs. Not all testers will.

Ground faults occur when some or all of the current returns by an alternate path, such as the ground wire. The green ground wire should not carry any current. It is like a safety valve in a hydraulic system. It provides an alternate current path to protect people and animals, just as a safety valve prevents a hose from bursting from overpressure.

You are most likely to see GFCI units around hot tubs and swimming pools and in bathrooms or other wet locations.

Specification Grade Receptacles. Specification grade receptacles are still available and cost more money, but are worth every penny when used in higher amperage, high-use locations. Look for SPEC stamped into one of the mount tangs of the receptacle. I wrote about them some years ago, but an excellent update appeared on page 33 of the Farm Building Promotional Supplement in the May 2008 issue of Better Farming.

Switch to vehicle/tractor wiring. The rules regarding wire colouring are NOT the same on equipment as in buildings. For example, white wires are often power wires, while ground wires are often black.

To determine wire colouring, you must often trace wire paths, which may require the wiring schematics.These may only be available in a service manual for that vehicle or unit.

However, a few important servicing rules apply. First, always make sure electrical connections are clean and tight. Battery terminals are the first thing you should check. They and other grounding terminals, where dissimilar metals are in contact, should always be protected by a dielectric compound to prevent corrosion.

If you are making electrical connections, do make use of a soldering gun or iron. Crimp-type connectors are only suitable for some low amperage circuits.

If any electrical connections have a high resistance to current flow, they will cause a voltage drop to occur. Newer electronic equipment will not work properly if the supply voltage goes too low during engine starts or as a result of wiring circuit voltage drops.

Significant voltage drops often cause heating, which can and does start fires. The fuse or breaker does not act quickly enough to open the circuit as it only responds to higher currents (those over the rating, i.e. 20 amps).

Just recently, we almost lost a 10-year-old vehicle to an electrical fire. The high amperage power-out line connector at the alternator had corroded. Unfortunately, it was out of sight because the alternator is located under the rear engine bank and is splashprotected by a metal shield on the underside.

The connector/alternator caught fire just after engine startup at the farmstead. Fortunately, the dog's water pail was close at hand. It would only have taken another few seconds to get a fire extinguisher from just inside the locked shop door, which might have been too long a time delay. Electrical fires progress very quickly!

In short, correct wiring techniques are essential for building or equipment wiring systems. High-resistance wiring connections can and do cause fires. True short circuits usually burn fuses or trip breakers when over current occurs. Hot connections often do not.

If you do your own wiring, it should be inspected. We in Ontario are fortunate that we can still legally do our own wiring. But do play it safe. If you do not understand electrical wiring systems, hire a contractor or mechanic who does. The life you save may be your own or that of a loved one. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.