Electrical Safety in the Laboratory

Every year, people are killed or injured while working with electrical circuits at home or at work. This document provides basic guidance for electrical safety in laboratories.

Background and Definitions

Current: The movement of electrical charge measured in ampere (amps).
Voltage: Measure of electrical force or potential difference in volts.
Watt: Unit of electric power, equals the voltage multiplied by current (W=V x A). 
Resistance: Opposition to current flow measured in ohms.
Conductors: Materials that have little resistance to electricity.
Insulators: Materials that have high resistance to electricity.
Grounding: A conductive connection to the earth serving as a sink for the current that is used as a protective measure against static electricity build-up.

The relation between voltage, current and resistance is given by Ohm’s law:
E (volts) = I (amps) x R (Ohms)

The three wires inside an electrical cord are color coded:
Black: Live AC voltage,
White: Neutral return,
Green: Ground, no current.



Injuries caused by electricity include electrical shock, burns, and falls due to electrical shocks and burns. Electrocution is a fatal electrical shock. Electrical shock occurs when current passes through the body. The severity of the shock depends on:

  • Amount of current flowing through the body,
  • Path of current through the body,
  • Length of time the body is in the circuit.

Note: Low voltage or low current does NOT mean low hazard! Less than 10 milliampere can cause a painful shock and loss of muscular control, and 50 milliampere can be fatal (see the table below for more details). 

Electricity seeks all paths and not just the path of least resistance to reach lower potential.



1 milliamp

Just a faint tingle.

5 milliamps

Slight shock felt. Disturbing, but not painful. Most people can "let go." However, strong involuntary movements can cause injuries.

6-25 milliamps (women)
9-30 milliamps (men)

Painful shock. Muscular control is lost. This is the range where "freezing currents" start. It may not be possible to "let go."

50-150 milliamps

Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Heart fibrillation possible. Death is possible.

1,000-4,300 milliamps (1-4.3 amps)

Rhythmic pumping action of the heart ceases. Muscular contraction and nerve damage occur; death likely.

10,000 milliamps (10 amps)

Cardiac arrest and severe burns occur. Death is probable.

15,000 milliamps (15 amps)

Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit!

*Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns.

Source: Kouwenhoven WB [1968]. Human Safety and Electrical Shock. Electrical Safety Practices, Monograph 112, Instrument Society of America, P. 93.


Fire Hazards

Too much current flowing through a wire can cause a power cord to overheat and start a fire. Sparks from electrical equipment can ignite flammable materials.

General Rules for Electrical Safety

  • Use electrical cords only if they are in good condition. Cords must not be cracked, frayed, or have corroded prongs.
  • Do not use 3-to-2 prong adapters unless other grounding provisions have been made. Plug 3-prong plugs into 3-prong outlets.
  • Use power strips that have circuit breakers or fuses. Do not link power strips in series. 
  • Do not leave cables and cords unsecured and hanging in areas where they can pose a trip and movement hazard. Place cords so that they are not subjected to mechanical stress or temperatures that could damage the insulation.
  • Do not conceal cords behind or attach electrical cords to building surfaces.
  • Do not leave electrical circuits exposed. Use electrical tape to insulate wires or use a guard as cover to prevent accidental contact.
  • Do not block access to electrical panels.
  • Do not install standard electrical equipment in locations where flammable gases, vapors, dusts, or other easily ignitable materials are present. If electrical equipment is used in a chemical fume hood, elevate it to allow efficient air flow.
  • Keep electrical equipment at a minimum in high-moisture areas (e.g., wash rooms, cold rooms).

Extension Cords

Extension cords are not a replacement for permanent wiring. Install outlets in areas where electricity is needed permanently.

Extension cords should NOT run through holes in walls, ceilings, floors, doors, or through windows and should be inspected before each use.

Use only 3-prong extension cords with a listing from Underwriters Laboratories (UL) or other reputable testing labs. 

Cord Ratings

Choose an extension cord appropriate for the current that will be flowing through it to avoid overheating. The required wire thickness depends on the power consumption of the equipment and the length of the extension cord. Check the equipment power requirements; calculate it by multiplying the voltage by the current. For example, a saw drawing a current of 5 amps at 120 volts requires 5 x 120 = 600 watts. Use an extension cord with a rating of at least 600 watts.

Extension cords have information regarding the wire size, construction type, and temperature range printed along the length of the cord. The size of the wires is given by the gauge. The lower the gauge number, the thicker the wire. The table below lists the minimum wire size for a given cord length and current as directed by the National Electrical Code:

NEC minimum wire size (gauge) for extension cord length

AmpsCord length (feet)
0 - 6 amps18 ga16 ga16 ga14 ga
10 amps18 ga16 ga14 ga12 ga
10 - 12 amps16 ga16 ga14 ga12 ga
12 - 16 amps14 ga12 gaNot recommended

Other information presented on the cord indicates the construction type and conditions for use, for example:  

S = Hard service cord (600V) 
SJ = Junior hard service cord (300V)
E = Thermoplastic elastomer insulation
T = Thermoplastic insulation
O = Oil resistant thermoplastic elastomer jacket
OO = Oil resistant thermoplastic elastomer jacket and insulation
X = Cross-linked polyolefin insulation
W = Damp and wet conditions

For a complete listing of codes for flexible cords and cables, see Articles 400 and 402 of the National Electrical Code (NEC).

Flexible cords must be rated for hard or extra-hard usage (S, ST, SO, STO).

Ground Fault Circuit Interrupters (GFCI)

A GFCI is an outlet device that senses current taking a wrong path (detects a grounding fault) and then disconnects the circuit to prevent electrical shocks. For example, a GFCI outlet trips (disconnects the circuit) when water splashes onto an operating heat gun. To protect users from electrical shocks caused by water, all electrical outlets within 6 feet of water must use GFCI protection (according to National Fire Protection Association 70 210.8 B5). Portable GFCIs are available if no GFCI wall outlet is present. If working in a wet area that is normally dry, a portable GFCI should be used.

Ground Fault Circuit Interrupter (GFCI) outlet

Equipment Fuses and Circuit Breakers

Fuses and circuit breakers are safety devices that protect equipment from high currents or voltages and prevent overheating of electrical wires. They are rated for a certain voltage and maximum current and come in two types: fast and slow blowing. When choosing fuses and circuit breakers, use appropriate amp rating (e.g., 10 amps, 15 amps) and type. Never replace blown fuses with fuses of higher ratings. Always disconnect the circuit or unplug equipment before inserting an in-line fuse. Never insert in-line fuses into a live circuit. If the new fuse blows again, determine the reason, or have the equipment checked by an electrician or the manufacturer.

Repairing Electrical Equipment

Only perform repairs referred to by the manufacturer's instructional manual. Any other work should only be performed by personnel certified by the manufacturer. 

  1. Turn the equipment off and leave it plugged in. Let it stand for a few minutes for capacitors to discharge.
  2. Once capacitors have had time to discharge, unplug the equipment.
  3. When servicing highly sensitive electronic components (e.g., electron multipliers or computer boards) that could be damaged by static electricity, ground yourself using an anti-static wrist band. Connect the wrist band to the ground.

Working with Electricity

Whenever possible, completely de-energize the system before performing any work. If work has to be performed on “hot” components, you need to be qualified for this type of work. Contact your school’s electronics shops or people responsible for facilities and services for help. 

Wear Personal Protective Equipment (PPE) and follow the techniques below when working with electricity:

  • NEVER wear rings, watches, bracelets, necklaces, or other electrically conductive jewelry.
  • Avoid being grounded. Stay at least 6 inches away from all metal materials, walls, and water sources. Wear shoes with thick, insulating soles or use non-conductive mats.
  • Probe hot wires and components with only one hand to prevent current from passing through your chest cavity and injuring your heart. Place the other hand at your side, in a pocket, or in a belt loop away from conducting materials.
  • Use tools designed for electrical work that have a non-conductive cover. Electrically insulated gloves are also available.
  • Use voltmeters with appropriate rating for the voltage to be tested. A standard voltmeter could explode when subjected to a high voltage.

Electrical Lockout-Tagout

Lockout-tagout is a safety procedure used by licensed electricians during repair and maintenance work to make sure electric power is disconnected and not turned back on before work is finished. NEVER remove a lockout-tagout device! This could endanger someone’s life. Examples of electrical lockout-tagout devices are shown below:


For training on electrical safety and electrical lockout-tagout, contact Safety and Compliance at 217-265-9828.


NRC (National Research Council). Prudent Practices in the Laboratory. Handling and Management of Chemical Hazards. National Academy Press: Washington, DC, 2011.

Krieger, G. R.; Montgomery, J. F. Accident Prevention Manual, 11th ed.; National Safety Council: Itasca, IL, 1997.

Last Update: 7/8/2015