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
). In addition, ECMA
, an industry standards association, published a useful treatise on Safety of electronic equipment
). A review article on Conduction of Electrical Current to and Through the Human Body
through the National Institutes of Health website. A plain-English chapter on Electrical Safety
appears in the Electronics Technician
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:
Under abnormal conditions, thresholds for the various hazards can be much lower.
- 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)
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.
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.
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.
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.
Personal Safety Protection
Wear proper safety protection when you work with electrical hazards. Gloves may be required (however, do not wear gloves when you are operating a revolving tool, such as a drill or saw). Safety glasses (prescription and non-prescription) and boots or shoes (of many types) are also available with purchase orders from the business office. Wear appropriate personal protective equipment when you are working with power tools, electricity of 50 volts or greater, or heavy equipment. Remember that watches and rings can cause fatal injury by acting as conductors. Remove all jewelry when you are working with power tools or near electrical hazards.
Know Your Equipment
Before beginning electrical work, check that your diagnostic equipment is in good working order, is properly calibrated, and covers the range necessary for the job. A small, hand-held digital voltage meter
(DVM) is commonly used, but you must check the rating on the front of the DVM to ensure that the voltage with which you are working is under the meter's maximum rating. It is dangerous to use a meter on voltages that exceed its maximum rating.
Use proper testing equipment and accessories when repairing AC or DC powered equipment. Choose probes and modules carefully because they are rated for different peak voltages. Remember that peak AC voltage is about one and a half times the normally quoted RMS (root mean square) voltage. For instance, the typical 480 volts RMS AC power exceeds the 600 peak voltage rating of some meters. If you are unsure of which probe to choose for the equipment you are planning to work on, then you should use a DVM (with a rating higher than the highest possible voltage in the equipment you are working on) to measure the voltage first.
When you are in damp or wet areas, you must power line-operated equipment through a ground fault circuit interrupter
(GFCI) . Test each GFCI before using it. Portable GFCIs are available from technical supervisors.
Before bench-testing a line-operated chassis, bond metal test benches to the ground and use a circuit breaker to ensure that the current is limited. A ground-fault circuit interupter may provide additional protection. You may also need a second person with you to satisfy the two-person rule
Before testing or working on high voltage equipment or equipment capable of energy storage, note any posted warnings and then de-energize it with a grounding stick
. The grounding stick, a metal wand with an insulated handle, is normally built in where needed (near outputs of DC power supplies, filter capacitors, etc.). Always disconnect the energy at its source before using a grounding stick, and, if possible, dissipate any stored energy. Be prepared to encounter stored energy when using a grounding stick. Wear safety glasses to protect your eyes from flying sparks while you securely attach the stick to the equipment. Keep the grounding stick in place so that voltages will not "float up", or come back, while you are working.
system is used to prevent equipment from being powered while someone is working on it. You must obtain the Wilson Lab LTV training session before using this procedure. Use the following steps when locking out machinery for maintenance:
- Announce the shutdown
- Shut off the equipment
- Isolate the equipment from its energy source
- Apply a lock to the disconnect or circuit breaker, and use a tag that states your name, the date, and the reason for the lockout
- Dissipate any stored energy
- Verify that the equipment will not operate, returning the controls to the off position
To restore power:
Never remove anyone else's lock and tag,
- Remove all tools and other loose objects from the area; reinstall any guards
- Notify all affected workers
- Verify all controls are off
- Remove the lock and tag and restore power
unless following the process documented in the EHS Lock Removal Permit
. To find out more about Lock/Tag/Verify, speak to your supervisor about attending a lockout/tagout training session.
See also the Cornell EHS
Lock/Tag/Verify (LTV) page
, and procedures
For emergency contacts, see Emergency Numbers
Common sense and the following basic guidelines will help you avoid serious harm in electrical emergencies involving fire, shock, or burns.
- Fire: See Fire Safety. If you see a fire, pull the nearest fire alarm. Switch off the power to the area if you can do so safely. In the CESR tunnel, it is often best to push a crash button or pull the crash cord to turn off power. If the fire is small and contained, and If you have been trained in fire extinguisher use, you may attempt to extinguish it; otherwise leave the area immediately and then notify the CESR Operator of the fire. If you attempt to put out a small, contained fire, use an extinguisher that is designated A-B-C, or B-C. (Never use just a Class A, or water extinguisher, on an electrical fire.) Always be on guard for the presence of hazardous voltages when extinguishing a fire. If the fire does not extinguish easily, leave the area and notify the CESR operator.
- Electrical Shock: If you find an unconscious person who is in contact with a power source, DO NOT, under any circumstances, touch the person or the wire itself. Disconnect the source of power first. Again, the best way to turn off equipment in the tunnel is to push a crash button or pull the crash cord. If you cannot turn off the source of power, then disconnect the victim from the source of power with a long, non-conductive object such as a broom or a rope. Designate a specific individual to call 911 and give the details of the accident. Once the unconscious person is disconnected from the source of power, use the ABCs of first aid to determine if the person is breathing. Stay with the victim, cover any wounds with sterile dressing, and wait for help to arrive.
- Burns: Treat electrical burns with care. If you or a co-worker has been burned, call 911 immediately for help. Refer to chapter ten for more emergency information.
End of Electrical Safety