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.

Electrical 101

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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:

V = A * R

A = V / R

R = V / A

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.

Electrical Safety Basics

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Verify that your test tool is working properly by checking a known-good voltage source.

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.

Other Dangers

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.

After the Work Is Done

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A receptacle tester with GFCI circuitry can identify reverse polarity wiring. Changes to that wiring should be made by a electrician.

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.

What To Watch Out For

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.

  • Knob and tube wiring: Bare copper wires stretched from one porcelain insulator to another.
  • Burned or partially melted insulation on wires: Wires damaged by electrical overload, a lightening strike, mis-wiring or even a fire in the room or an adjacent room.
  • Burned or partially melted contacts. The wiring may be fine but a lightning strike, a loose connection, moisture, and electrical tinkering can all damage the point at which the wiring connects to the switch or outlet. Any blackening or a brown shade on the switch or outlet is cause for concern. You'll need a licensed electrician to investigate.
  • Evidence of water leakage in your service panel. If you go to flip a circuit breaker and you see water stains on the panel or around it, don't go any further. Call an electrician. It's not unusual in older houses to find that water has leaked into the service panel due to a poor seal at the electrical meter.
  • Evidence of obviously sloppy electrical workmanship. This is self explanatory. Dangerously handy homeowners and various kinds of electrical slobs have been known to use coffee cans rather than electrical boxes and make electrical splices using duct tape, to cite examples that routinely turn up in home inspections and remodeling work. Sure it'll work. It can also burn your house down or lead to an electrocution.
  • Repeated tripping of circuit breakers and GFCI devices, a few hours or even minutes apart. The occasional trip is one thing. Multiple trips during a day or week are cause for concern.
  • Circuit breakers tripping consistently at certain times of day. If you trip a circuit breaker every time you run a hair dryer or a vacuum cleaner, that's a clear warning that something is amiss. Call an electrician.
  • A circuit breaker that trips immediately after being reset. Obviously, that's a bad sign and indicates an electrical problem that a homeowner should not attempt to fix. It could be an electrical load plugged into the circuit, faulting wiring, or the circuit breaker itself.

How To Use a Digital Multimeter

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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:

  • DC voltage: Test the voltage of batteries used in portable devices.
  • Light bulb continuity: Contrary to popular opinion, incandescent bulbs are still available and in some cases are preferable to LED or CFL models. Some flashlights will only accept a small low-wattage incandescent bulb. You can't always see whether these bulbs have a broken filament, but a DMM will find it every time. A broken filament means that the continuity (or continuous electrical path) through the bulb is broken.
  • Solenoid continuity: Many appliances that use water, such as a dishwasher or a refrigerator with an ice maker, will have a solenoid valve, (a valve opened and closed by a small electro magnet). A small break in the solenoid's electrical coil will cause the valve to stop working. You can't see the break, but checking continuity with a DMM will tell you whether you need to replace the solenoid.

Finally, a Safety Checklist

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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.

Don't work alone.

When in doubt, ALWAYS call an electrician.

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Roy Berendsohn
Senior Home Editor

Roy Berendsohn has worked for more than 25 years at Popular Mechanics, where he has written on carpentry, masonry, painting, plumbing, electrical, woodworking, blacksmithing, welding, lawn care, chainsaw use, and outdoor power equipment. When he’s not working on his own house, he volunteers with Sovereign Grace Church doing home repair for families in rural, suburban and urban locations throughout central and southern New Jersey.