Electrical and electromagnetic safety and interactions in biomedical devices

Electrical safety

Hazard recognition (non biomedical)

• Cords and equipment: Power tools and extension cords must be inspected each time they are used. They must be taken out of service immediately upon discovery of worn or broken insulation. Visually inspect electrical equipment looking for damage and/or external defects such as loose, missing, or deformed parts or damaged insulation. Often external damage may indicate internal damage to the equipment. Before cleaning electrical equipment, turn it off and unplug it.

• Electrical panels must be kept clear of any obstructions at all times so storage is not allowed
• Trip hazards: A temporary cord has to be covered to protect other workers

• Exposed wiring: Assume that all exposed wirings are energized, we have to turn off the current, protect the area and call supervision

• Power strips: No plug multi-outlet strips onto each other
• Junction boxes, pull boxes fittings and cabinets: Must be covered or closed

Effects of an electric shock on the human body

Skin resistance Z ranges from 10 kΩ to 1 MΩ (per cm squared²). If skin is wet or broken, Z may drop to 1% of its original value. Research shows that minimal let-go current occurs for commercial power line frequencies: 50 and 60Hz. The let-go current (or release current) is the maximum value of electric current through the body of a person at which that person can release himself or herself.

When a person becomes part of the circuit touching a wire, the current enters the body from the wire and leaves it from the earth. We can have direct and indirect (falls) effects:

• Electrocution

• Electrical shock: Received when current passes through the body. Severity of the shock depends on:

  1. Path of current through the body.
  2. Amount of current flowing through the body.
  3. Length of time the body is in the circuit.

  The longer the exposure, the increased danger of shock to the victim. Low voltage can be extremely dangerous because the degree of injury depends not only on the current but on the length of time in contact with the circuit. (Example: A current of 100mA applied for 3 seconds is as dangerous as 900mA applied for 0.03 seconds). High voltages lead to additional injuries such as violent muscular contractions. Muscle contractions may cause bone fractures from either contractions themselves or from falls. Internal bleeding, destruction of tissues, nerves, and muscles.

• Burns: The most common shock-related, nonfatal injury is a burn. Burns caused by electricity may be of three types:

  1. Electrical burns: Electrical burns need to be given immediate medical attention. Electrical burns occur when a person touches electrical wiring or equipment that is used or maintained improperly. Typically such burns occur on the hands. Clothing may catch on fire and a thermal burn may result from the heat of the fire.
  2. Arc burns: An arc-blast is a luminous electrical discharge that occurs when high voltages exist across a gap between conductors and current travels through the air. Temperatures as high as 19500°C have been reached in arc-blasts. The three primary hazards associated with an arc blast are: thermal radiation (heat) and intense light which causes burns, a considerable pressure wave blast, may cause copper and aluminium components to melt.
  3. Thermal contact burns

• Falls: Electric shock can also cause indirect injuries. Workers in elevated locations who experience a shock may fall, resulting in serious injury or death.

Electrical hazard protection

Insulation: Is made by a plastic or rubber covering that does not conduct electricity, prevents live wires from coming in contact with people and protecting them from electrical shock.

Grounding: “Ground” refers to a conductive body, usually the earth, and means a conductive connection, whether intentional or accidental, by which an electric circuit or equipment is connected to earth or the ground plane. By "grounding" a tool or electrical system, a low resistance path to the earth is intentionally created. It is used to guarantee the electric masses have the same potential as the earth so that they disperse in the ground without hitting the man. It is therefore necessary to connect every part of a building to a standard earthing system. Sometimes they work in synergy with differential switch or circuit breaker.

• Guarding is a type of isolation that uses various structures to close off live electrical parts, including boxes, screens, covers, and partitions.
• Electrical protective devices (GFCI ground fault circuit interrupter) or Fuses and circuit breaker
• Personal protective Equipment (foot protection, head protection, hand protection) and safe work conditions, plan work, do not work in wet conditions, use proper wiring and connectors

Ground Fault Circuit Interrupter (GFCI)

A ground fault circuit interrupter (GFCI) can help prevent electrocution. It is a type of circuit breaker which shuts off electric power when it senses an imbalance between the outgoing and incoming current. This difference in current could happen when electrical equipment is not working correctly. If a person’s body starts to receive a shock, the GFCI senses this and cuts off the power before he/she can get injured. GFCIs are generally installed where electrical circuits may accidentally come into contact with water. They are most often found in kitchens, baths, and laundry rooms, or even out-of-doors or in the garage where electric power tools might be used.

A “ground fault” is a conducting connection (whether intentional or accidental) between any electric conductor and any conducting material that is grounded or that may become grounded. Electricity always wants to find a path to the ground. In a ground fault, electricity has found a path to ground, but it is a path the electricity was never intended to be on, such as through a person’s body. Because of this potential for shock, GFCI protection is used to protect human life. A GFCI constantly monitors current flowing through a circuit with a differential transformer to compare the going out current on the hot wire with coming back current on the neutral. If it differs by a very small amount, as 5mA, from the returning current, the GFCI interrupts power really fast to prevent a lethal dose of electricity, using an internal solenoid, that mechanically triggers the circuit breaker. When the problem is fixed, the user can reset the GFCI by pushing the reset button. There is also a test button which will cause the GFCI to trip if it is working properly. Should be tested once a month!

GFCIs are available in 2 types: the circuit breaker, installed in an electrical panel; and the receptacle type, installed into an electrical box.

Microshock and Macroshock

Microshock: Is the result of very small currents applied directly to the myocardium. They are electrical shocks causing ventricular fibrillation when currents enter into a patient's body when it is subjected to medical procedures involving intracardiac electrical conductors, such as an external pacemaker electrodes or saline-filled catheters. The cause is leakage current in equipment, undesired currents through insulated conductors at different potentials (if low resistance ground is available there is no problem, if ground is broken there is current flow through the patient) or differences in voltage between grounded conductive surfaces. The risks increase when the patient is subjected to medical surgical practices involving cardiac catheterization procedures or, more simply, the application of probes or electrodes near the heart. The accepted safety limit to prevent them is 10μA.

Macroshock: Are the effects of an electric current on the body which can lead to severe injury or death. The macroshock occurs when there is a passage of current in the person due to the contact between an accidentally live part and a part of the human body. The current passes through the body affecting a large section but only a small part flows through the heart. The risk of the heart entering ventricular fibrillation is minimal. In fact, when current is applied at two points on the surface of the body, only a small fraction of the total current flow is through the heart. If we want to act on the heart, we have to use an intracardiac catheter. In general, the greater the current, the more dangerous a shock is and the more likely it is to be lethal. A high-voltage, low-current shock is not dangerous, but a low-voltage, high-current shock may cause significant harm or death.

Classification of medical equipment

• Class I: On this equipment, we have the basic insulation but also all the accessible conductive parts are connected to a protective earth conductor so these parts cannot become live in case of basic insulation. This conductor has to be connected between the protective earth terminal and external protective earthing system, the max resistance has to be 0.2 Ohm, instead for flexible detachable supply cables 0.1 Ohm.
• Class II: We have two layers of insulation between any live part and accessible parts of equipment, they are identified by a square within a square symbol. Classes I and II relate to equipment operating at mains voltage.
• Class III: The protection against electric shock relies on supply at safe extra-low voltage (SELV) and in which