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Everything you need to know before you work around the wires in your home.

Getting shocked is more than unpleasant. It can be lethal. An electrical impulse as small as 14 milliamps is enough to kill a person. The current drawn by a plug-in nightlight is more than enough to do the job.

I was thinking about these grim facts not long ago while doing some remodeling in an old house. I had to remove the plaster wall surface that had ugly paneling glued to it. That meant exposing live wires running inside the wall, which meant I had to cut power to that circuit before I could work safely. Even though this wasn't "electrical work," it led me to think about the times I've been zapped or come close to getting zapped while remodeling.

We encourage readers to try all kinds of DIY projects. Being handy with electrical work can help you keep your home in tip-top shape, save money by repairing appliances, and avoid household dangers. But look, you've got to be serious about safety if you're going to start messing with electricity. That's why I decided to do this primer about how to avoid electrical shock.

An electrical impulse as small as 14 milliamps is enough to kill a person. The current drawn by a plug-in nightlight is more than enough to do the job.

The first thing I did was to call my friends at Fluke. We like their electrical test instruments and use their meters exclusively when taking readings during our product tests. We use their power quality analyzer whenever we test generators and we like the company's 233 meter for doing routine electrical checks. Fluke's engineers and technicians are experts in electrical safety because they supply meters and test instruments to electricians who have to work under extremely dangerous live voltage conditions. They were kind enough to help us out with this guide.

First you have to understand alternating current (AC) electricity at a basic level. This is the kind of electricity that the utility supplies to your house.In very simple terms, voltage is the force or pressure that moves electrons through a conductor. Current is the number of electrons moving through the conductor. Resistance is the opposition to movement of electrons in a conductor.

Imagine a circuit containing a source of AC electricity (say, a power outlet), a load (for example, a toaster) that offers resistance, and a device that measures the current through the load. It could be an AC ammeter, or a digital multimeter (DMM). Let V stand for the voltage, A stand for the current through the load (the number of amps being pushed by the voltage), and let R stand for the resistance of the load (in ohms). Three related formulas, as defined by Ohm's Law for AC:

In the example above, V would be 120V AC, since that's the voltage standard across the U.S. The resistance of a typical toaster's heating element is approximately 12 ohms. Thus the current through the toaster is approximately 10 amps. To keep things in perspective, a current of 10 milliamps (mA), which is just .001 of the current in a toaster, can cause painful, uncontrolled muscle spasms, while 20 mA can cause heart fibrillation.

Before you begin any work like the remodeling project I describe above, you should reach for your non-contact voltage tester (NCVT). The electro-magnetic field around a live wire will cause this tool to beep and light up, showing the presence of AC voltage. You don't have to make contact with an energized conductor for it to work, either. These instruments are so sensitive that they emit their signal as you draw close to the voltage source, such as a switch, outlet, or energized wire.

The first step is to prove to yourself that your tools are working properly. For an NCVT, verify operation by testing on a known-live circuit (see photo).

Next,turn off the circuit breaker. Then check one of the outlets on that circuit with the NCVT. Especially in an old house, don't assume all wires in a box are dead even when the circuit breaker is off. Why? Sometimes a handyman has used an electrical box as a junction box and brought wires into it from another circuit breaker. Those wires will remain energized and could shock you.

After the wall surface had been stripped during my remodeling project, I used my NCVT to check that all wires in the wall were dead. As it turned out, they were not. I needed to turn off a second circuit breaker. The lesson to here is not to assume. The outlet was dead but nearby wiring was not, as the NCVT soon proved.

Here's another old house danger you might not expect: reverse polarity. Bad wiring can cause wires to be energized when they normally shouldn't be.

You can test for this in an outlet by using a receptacle tester with GFCI (such as Amprobe ST-102B Receptacle Tester). The lights on this device illuminate in a pattern that will tell you if you have a mis-wired outlet. Note that in rare instances, even these devices can be fooled. An excellent report on that phenomenon is contained in this article by Electrical Construction & Maintenance.

If you find faulty wiring or any kind of damage, repairs or alterations should be made by a qualified electrician before any further testing or remodeling work is done.

When remodeling work is complete and power is restored, conduct a few basic tests to ensure that switches and outlets are in working order. Use the receptacle tester on outlets and check switches with the non-contact voltage tester before placing any kind of an electrical load on the circuit.

If you're testing an outlet that is powered by a wall switch (known as a switched outlet, which is a common means of powering a floor lamp, for example), you can use the non-contact voltage detector to check that the outlet is live when the switch is in the "on" position.

It's not unusual in remodeling work to expose dangerous or damaged wiring or something that looks suspicious. If you uncover anything on the list below, it could be dangerous or potentially lethal. So you need to call an electrician before proceeding. Here are a few trouble spots to look out for.

A digital multimeter (DMM), especially one with a NCVT feature, belongs in the tool kit of anybody who does remodeling, repair and maintenance around the home. These battery-powered instruments perform a variety of electrical tests and will help you keep your home and appliances running smoothly.

A partial list of some of the things you can test with a DMM are these:

Use an electrical test tool that meets accepted safety standards for the environment in which it will be used. This is usually not a problem for homeowners but something that professionals have to be aware of. For example, they won't test high-voltage electrical equipment unless they have a meter or device rated for that. Still, homeowners would do well to use a meter or a NCVT with a rating by UL or other accredited testing organization.

Use only test leads that have shrouded connectors and finger guards.

Inspect the test leads of a DMM for physical damage before making a measurement.

Use the DMM to check continuity of its own the test leads.

Use only test leads that have shrouded connectors and finger guards.

Use only meters with recessed input jacks.

Be certain that electrical test tools are in good operating condition.

Follow all equipment safety procedures as called for by the manufacturer.

When in doubt, ALWAYS call an electrician.