CLASSE Safety Handbook

Electrical Safety

Material here is for general electrical information only. Additional training and authorization is required before working on high voltage/power equipment.

There is valuable information at the EHS Electrical Safety pages, especially a document detailing electrical hazard evaluation and electrical training guidance: Electrical Safe Work Practices (pdf). In addition, ECMA, an industry standards association, published a useful treatise on Safety of electronic equipment (pdf). A review article on Conduction of Electrical Current to and Through the Human Body is available through the National Institutes of Health website. A plain-English chapter on Electrical Safety appears in the Electronics Technician journal.

Electrical Hazards

Electricity is an important tool, but it can be dangerous. Whether you sit at a computer terminal or rewire circuits in the tunnel, you are at risk of becoming the path of an electrical current. Simple safety practices such as checking to see that the work area is dry and uncluttered, an outlet is not overloaded, or that a cord is not frayed may save you from fire, shock, or electrical burns.

Under normal conditions, any exposed conducting surface should be considered hazardous if the voltage is higher than 50 V. Note that CLASSE has adopted the Cornell EHS level of 50 V as defining hazardous voltage, but voltage alone is far from the whole story: circuits with low voltages can sometimes deliver hazardous current and/or stored energy. For example, superconducting magnets can operate with low voltage drops but high current and high stored energy, and low voltage buses can sometimes be capable of delivering high current. Due to high capacitance or inductance, even disconnected circuits can have high stored energy. Moreover, abnormal conditions (e.g. the contact area with or path taken through the body, ambient humidity, individual perspiration level, the presence of water on or near the circuit) can increase the hazard level for any given voltage or current. For DC voltages, currents above 25 mA at 50 V are considered hazardous under normal conditions. For common AC outlets (60 Hz, 120 V), current above 10 mA is considered hazardous under normal conditions.

The health consequences of contact with common household outlet (120V, 60 hertz) AC current, under normal conditions, versus current (in milliAmperes, or mA) can include:

• 1 mA: Usually harmless, but slight tingling or involuntary movement from the shock can cause injury.
• > 5 mA: Painful shock. Possible loss of muscle control.
• >10 mA: Victim may be unable to let go of source.
• >20 mA: Severe shock, nearly certain loss of muscle control, difficulty breathing.
• >75 mA: Possibly fatal.
• 15000 mA: Common fuse breaker opens circuit (i.e. too late to save a person)

Under abnormal conditions, thresholds for the various hazards can be much lower. In addition to risks of restricted respiration, cardiac arrest, and internal organ damage, conducting high currents through the skin can leave severe burns.

It is always prudent to use one or more safeguards (such as wearing appropriate protective equipment, or de-energizing a circuit before working on it) if the hazard level is uncertain. However, if the 50 V level is exceeded, or abnormal conditions prevail, the use of safeguards is required, as is specialized training.

Another hazard of some electrical equipment is that of "arc flash". An arc flash occurs when an unintended low impedance current path becomes available to a circuit capable of supplying high current, and results in an explosion which blasts molten metal and an expanding plasma with great force and extremely high temperatures. Serious injury and death can occur for anyone in the vicinity of an arc flash, especially if appropriate protective equipment is not worn. An arc-flash hazard may exist at some AC power distribution panels, with the danger being highest for higher voltages and power capabilities. Specialized training is necessary to work in the vicinity of any arc-flash hazard.

Good Housekeeping

Poor housekeeping is the main reason that electricity is the number one cause of fires in the workplace nationwide. Although safety features such as circuit breakers, ground fault circuit interrupters, and insulators are designed to prevent electricity from becoming the heat source for fires, poor housekeeping may still cause a fire. Good housekeeping requires that you:

• Clean up liquid spills near electrical equipment immediately and be sure that electrical equipment is thoroughly dry before applying power
• Store flammable liquids away from electrical equipment
• Keep electrical cords away from heat sources
• Do not overload outlets
• Do not plug a power strip into another power strip
• Keep paths to fire extinguishers clear

Keep your work area neat and clean to protect yourself and your co-workers from fire hazards, but remember your own safety depends on good housekeeping throughout the lab. If you see faulty equipment or poor housekeeping anywhere in the lab, please report it immediately.

Two-Person Rule

Never work alone when you could be exposed to hazardous voltages (above 50V) or current (above 25 mA). A second person must stand as a backup. The backup person must be in the immediate area and attentive to the work in progress. He or she must know both where to shut off power and how to call for help. If you are unsure whether you qualify to act in a role of the second person, then consult your supervisor. He or she is responsible for assuring each person's qualifications, and will assign one person to do the work and the other to be the backup.

Extension Cords

Damaged and improperly used extension cords cause thousands of electrical fires and injuries in homes and workplaces every year. Before using extension cords, inspect them for cracks, ripped insulation, and damaged or missing grounding prongs. Do not bend back or break off grounding prongs: they are important safety features that receive harmful stray currents. To prevent overheating or possible electrical shock, choose cords rated for current levels that exceed your needs by a safe margin. Never run extension cords through walls, windows, or doorways or behind walls, ceilings, or floors. Never connect extension cords in a series or splice them together. Remove the extension cord when you are finished. Extension cords are not to become permanent fixtures for regular use.

If you are setting up or adding new equipment in an area without adequate outlets, the CLASSE Facilities Engineer can arrange to have capacity added.

End of Electrical Safety