Main questions

Microshock and Macroshock

Microshock

Microshock is the result of very small currents applied directly to the myocardium. It refers to the risk that patients undergoing medical procedures involving externally protruding intracardiac electrical conductors, such as an external pacemaker electrodes or saline-filled catheters, could suffer an electric shock causing ventricular fibrillation due to currents entering the body via these parts.

Causes of Microshock

• Leakage current in equipment causing undesired currents through insulated conductors at different potentials; if low resistance ground is available, there is no problem, but if ground is broken, there is current flow through the patient.
• Differences in voltage between grounded conductive surfaces.

The microshock can be due to the operator and due to the surrounding equipment. 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. Small currents called microshock can induce ventricle fibrillation (20 µA). The accepted safety limit to prevent them is 10 µA.

Macroshock

Macroshock is a medical term for the effects of body exposure to electrical current, which can lead to severe injury or death by electrocution. 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 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. 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. The most probable cause of death is ventricular fibrillation.

The magnitude of the current needed to fibrillate the heart is greater when the current is applied on the surface of the body than it would be if it was applied directly to the heart. All the current applied through an intracardiac catheter flow is through the heart.

Ground Fault Circuit Interrupter (GFCI)

A ground fault circuit interrupter (GFCI), or Residual Current Device (RCD), is a type of circuit breaker that 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. The main purpose is to protect people (not equipment) from an electric shock caused when some of the current travels through a person's body due to an electrical fault such as a short circuit, insulation failure, or equipment malfunction. Standard circuit breakers shut off power when the current is too high; GFCI is set at about 5 mA.

Functions of GFCI

• Prevents fires from short circuits and other electrical faults that don’t involve humans.

A ground fault is an unintentional electrical path between a power source and a grounded surface. It occurs usually when equipment is damaged or defective, such that live electrical parts are not protected from contact. If your body provides a path to the ground for this current, you could be burned, shocked, or electrocuted.

A GFCI constantly monitors current flowing through a circuit. If the current flowing into the circuit differs by a very small amount, as 5 mA, from the returning current, the GFCI interrupts power really fast to prevent a lethal dose of electricity, using an internal solenoid, which mechanically trips the built-in circuit breaker. The GFCI uses a differential transformer to compare the going out current on the hot wire with the coming back current on the neutral.

If a GFCI device trips and the fault is later fixed, the user can reset the GFCI by pushing the reset button. There is also a test button that will cause the GFCI to trip if it is working properly. It should be tested once a month! Typical value of leakage current = 5 mA.

Types of GFCI

• The circuit breaker, installed in an electrical panel.
• The receptacle type, installed into an electrical box.

Insulated Power Supply

Electrical Hazard Protections

• Insulation: Plastic or rubber coverings that do not conduct electricity. Insulation prevents live wires from coming in contact with people, thus protecting them from electrical shock.
• Grounding: Grounding is another method of protecting you from electric shock. However, it is normally a secondary protective measure. The "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.
• Guarding; Electrical protective devices; Personal Protective Equipment Safe work practices

Isolated power systems are designed to protect patients and personnel from electric shock in care areas, maintain the continuity of power in the case of a first line-to-ground fault, and continuously monitor the cumulative hazard current from all connected equipment. If there is a fault, the system alarm in the isolation panel activates, and the critical medical equipment remains operational because no ground fault protection or overcurrent protective device trips. The triggering of an alarm from a single ground fault must be rectified as soon as possible at a safe time, as a second ground fault could trigger the short circuit protection and take an entire operating room offline.

The IPS is a system comprising an isolated transformer, a line isolation monitor (LIM), and its ungrounded circuit conductors. The LIM is a test instrument designed to continually check the balance and unbalance impedance from each line of an isolated circuit to ground and equipped with a built-in test circuit to exercise the alarm without adding to the leakage current hazard.

Advantages

• No macroshock hazards for a single fault condition.
• No sparks with a single fault condition.
• A single fault does not affect performance.

Disadvantages

• If there is a single fault condition, problems may last for a long time before being detected.
• The system becomes uninsulated, and if there is another fault, heavy current will flow.

Electrical Measurement on Biomedical Devices

Applied Part or Patient Leakage is the most important part of the leakage measurement on any medical device. Applied Parts are directly in contact with the patient and are in the case of invasive devices placed under the patient's skin, which forms our natural protection against electrical currents. Currents applied under the skin can result in far greater consequences. Currents as low as 15 μA can result in fatality.

The limits for leakage currents within the IEC 60601-1 requirements are set to minimize the probability of ventricular fibrillation to a factor as low as 0.002 (Limit of 10 μA for CF Applied Part under normal conditions).

Safety Verification

It is important to verify that a medical device with moving parts (e.g., motor or pump) is safely mounted to allow movement without causing damage to equipment or personnel. Secondary Earth paths will affect the leakage measurements and might give false PASS readings. Always make sure that the device under test is positioned safely and isolated from Earth when measuring leakage.

To ensure a traceable simulation of current as if passing through a human body, measurement circuits have been designed to simulate the average typical electrical characteristics of the human body. These measurement circuits are referred to as Body Models or Measuring Device (MD in IEC 60601-1).

IEC 60601-1 specifies that all leakage measurements should be carried out using normal and single fault conditions. A typical part of the electrical safety testing procedures is to perform the test as follows:

• Normal Supply Voltage No (SFC)
• Normal Supply Voltage Open Neutral
• Normal Supply Voltage Open Earth
• Reversed Supply Voltage No (SFC)
• Reversed Supply Voltage Open Neutral
• Reversed Supply Voltage Open Earth

Earth Leakage Test

The Earth Leakage Test shows the current flowing through or via the insulation of the medical device into the protective Earth conductor.

• Earth Leakage, normal conditions - This test measures the Earth Leakage current under normal conditions. The current is measured through the Measuring Device with S1 closed and S5 normal and then S5 reversed.
• Earth Leakage, single fault, supply open – This test measures the Earth Leakage current with a single fault condition (supply open). The current is measured through the Measuring Device with S1 open and S5 normal and then S5 reversed.

Enclosure Leakage

Enclosure Leakage displays the current that would flow if a person came into contact with the housing (or any accessible part not intended for treatment or care) of the medical device.

• Enclosure Leakage, normal condition - This test measures the enclosure leakage current under normal conditions. The current is measured through the Measuring Device with S1 and S8 closed and S5 normal.