fast facts 

Electrical Impedance 

Hazards 

May 2009 

Office of Compliance 

advancing safety, health, and workplace rights in the legislative branch 

www.compliance.gov 

Impedance, in very simple terms, is resistance 

occurring in AC (as opposed to DC) electrical 

circuits. The degree of resistance is measured in 

units called ohms. The lower the impedance, the 

easier it is for electricity to flow through an 

object. If electricity can follow either a path with 

low impedance, such as a ground circuit, or high 

impedance, such as a person with dry hands, then 

the majority of the electricity will follow the path 

with low impedance. Electrical circuits are made 

up of several different parts. Impedance can 

occur in the hot wire portion of a circuit, the neutral side of a circuit, or in the ground conductor 

wiring portion of a circuit. This Fast Facts 

focuses on ground wiring circuits typically found 

in wall outlets (namely circuits that are 115 volts 

AC with between 15 and 20 amps of current). 

More specifically, it will examine the grounding 

conductor, located inside an electrical outlet, and 

the portion of the circuit between the outlet and 

the panel board or breaker box. 

Grounding Conductors 

Grounding conductors have very low impedance. 

Many electrical devices such as motors, ovens, 

and computers are designed so that if a fault 

(short) occurs and energizes something that is not 

meant to be energized, such as the frame of a 

computer monitor, then an employee would not 

be shocked by touching the energized object. To 

prevent electrical shocks or electrocutions, electrical equipment designers install ground circuits 

in products. The grounding circuit (or “third 

wire” on electrical plugs) safely conducts the 

unwanted fault current to ground. Instead of 

shocking the person touching the energized 

object, most of the current follows the low resistance path to ground. 

 

The Path of Least Resistance 

The cliché about electricity following the path of 

least resistance is incorrect. Electricity will follow 

all paths back to the source, but it will follow paths 

proportionally according to the conductivity of each 

path. When wall outlet ground circuits have very 

low impedance compared to a person in contact 

with an energized object, they can safely convey 

high current away from a faulty electrical device 

and trip the circuit breaker or fuse in most instances 

(See figure 1). However, when impedance 

increases even slightly, the electrical current seeks 

other paths to ground other than the path contained 

in the grounding circuit of a wall outlet, such as 

through a person in contact with an energized surface. Different factors can influence whether an 

employee may receive a serious shock when touching an energized device. Wet hands can lower a 

body’s impedance. As a result, a person’s body can 

become a better electrical conductor than a wall 

outlet ground circuit with higher impedance, and 

the person could receive an electrical shock. As the 

current going through a person’s body increases, so 

does the potential for serious harm. Studies have 

shown that as few as 10 milliamps on a 115 volt 

system can stop a person’s heart. 

Figure 1: Impedance Tester
fast stats 

• In 2007, the Bureau of Labor Statistics reported that 1,100 people in the US suffered burn injuries due to electrical 

hazards. 

• During 2007, the Bureau of Labor Statistics also reported that 1,480 people suffered electrical shock injuries. 

• The National Institute for Occupational Safety and Health (NIOSH) reports that electrocution is the third leading 

cause of worker fatalities in the United States. 

 

 

This information does not constitute advice or an official ruling of the Office of Compliance or the Board 

of Directors and is intended for educational purposes only. For further information, please refer to the 

Congressional Accountability Act (2 U.S.C. 1301 et seq.) and the regulations issued by the Board, or you 

may contact the Office of Compliance.

The Office of Compliance advances safety, 

health, and workplace rights in the U.S. Congress and the Legislative Branch. Established 

as an independent agency by the Congressional Accountability Act of 1995, the Office 

educates employees and employing offices 

about their rights and responsibilities under 

the Act, provides an impartial dispute resolution process, and investigates and remedies 

violations of the Act. 

If you have any questions, please do not hesitate to 

contact the Office of Compliance: 

Room LA 200, John Adams Building 

110 Second Street, SE 

Washington, D.C. 20540 

t/ 202-724-9250 

tdd/ 202-426-1912 

f/ 202-426-1913 

Peter Ames Eveleth 

General Counsel 

Mary-Margaret 

Smith 

Editor 

Download the entire Fast Facts suite at http://www.compliance.gov 

Causes of Impedance 

What causes impedance problems on a grounding circuit? On Capitol Hill, we have found that a poor contact 

between two electrical conductors is usually to blame. This occurs most often at the outlet itself. Sometimes the 

screw fastening the ground wire conductor in the receptacle needs to be tightened. Sometimes the outlet itself is 

broken or worn so much that it needs to be replaced (See figure 2). Sometimes the screw fastening the ground 

wire in the breaker box needs to be tightened. In rare cases, the distance between the breaker box and the outlet 

is too great and needs to be shortened by re-wiring the outlet to a closer box. Aluminum wiring, as opposed to 

copper wiring, can readily lead to impedance problems. Fortunately, aluminum wiring is no longer used in Legislative Branch buildings. 

OSHA’s electrical regulations require that circuits that could be used on sensitive electrical equipment, such as 

computers, should have an effective ground with sufficiently low impedance. See

29 CFR 1910.304(b)(2)(ii); see also 29 CFR 1910.399 (defining same). The most 

recent National Electrical Code defines “effectively grounded” as “connections of 

sufficiently low impedance and having sufficient current carrying-capacity to prevent the building up of voltages that may result in undue hazards to connected 

equipment or to persons.” OSHA uses the same definition. Electrical safety engineers have calculated that impedance levels in ground circuits must remain less 

than 1 ohm. Commercial testing equipment is available for measuring the amount 

of impedance in electrical receptacles. Inspections identify outlets with ground 

impedance levels higher than 1 ohm as potential safety hazards that need attention 

from a qualified electrical technician. 

Figure 2: Broken Outlet