Electrical and electromagnetic safety and interactions in biomedical devices

Types of Medical Equipment

• Type B: Class I, II or III. Adequate protection against electric shock with regards to leakage current and reliability. Suitable for external use and internal applications except catheterization.
• Type BF: Floating isolated applied part. It is only intended for connection to patient's skin but has floating input circuits. No connections between patient and earth.
• Type CF: Class I, II or III providing a higher protection against shock intended for direct cardiac applications. Minimum required resistance between mains lead and earth = 20 MΩ and 70 MΩ between mains leads and applied parts connected to patient.

Electrical Configuration of an Hospital Room (Equipment design)

• Reliable grounding of equipment: A low resistance ground wire should be connected between case and receptacle. Stair relief devices are recommended. Avoid 3-2 adapters.
• Reduction of leakage current: Special low-leakage power cords are available (<1 μA). Inside case,

leakage current is reduced using insulation materials which minimize capacitance between the live wires and case (chassis).

Operation at low voltage: Since almost all electronic circuits are operated at low DC voltages, Macroshocks can be avoided if supply voltage is low. However, Microshock is still possible but still safer.

Driven-right-leg circuit: Refer to ECG system design. This circuit plays a further role in isolating patients from earth by a very large resistor (5 MΩ), thereby limiting current to very small values (1 μA).

Disegno slide 22 Electrical Safety medical equipment

Use of current limiters

Electric isolation of patient circuits: amplifiers provide patient isolation from ground while still faithfully transmitting signals across the isolation barriers.

Equipotential grounding: Low resistance ground that any currents up to circuit breaker. Keep all conductors & surfaces & receptacles grounds at the same potential.

Disegno slide 24 Electrical Safety medical

Equipment Ground Faults Circuits Interrupters (GFCI)

Proper wiring distribution and grounding

Line isolation system and monitors

INSULATED POWER SYSTEM

There are different ways for Electrical Hazard Protection, we can have the insulation, so plastic or rubber coverings that does not conduct electricity, we can have grounding, so the intentional creation of a conductive low resistance path that, in which currents flow from the live part of the circuit to ground and other electrical protective device and safe work condition.

Isolated power systems are designed to protect patients and personnel from electric shock in care areas, maintains 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.

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.

• Advantages: no macroshock hazards and no sparks with single fault condition and 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 and the system becomes uninsulated and if there is another fault heavy current will flow

To solve this problem, line isolation monitors are used. It monitors impedance between either lines (A&B) and earth. If current through impedance goes > 2 mA, an alarm (audio or visual) is given (don't over react).

• EL.6 ELECTRICAL CONFIGURATION OF AN OPERATING ROOM

Operating rooms shall be considered to be wet procedure locations,

provided with ground fault circuit interrupters (GFCIs) to protect against electric shock. GFCIs monitor the flow of electricity and quickly shut off power if a ground fault is detected. This helps to prevent electrical shocks and potential injuries. In addition to GFCIs, wet procedure locations must also have special protections in place. These areas are where procedures are performed in wet conditions, such as when there is blood or fluids on the floor or table. To ensure safety, each operating room must have a minimum of 36 receptacles. At least 12 of these receptacles can be connected to either the normal system branch circuit or a critical branch circuit supplied by a different transfer switch. To further reduce the risk of electric shock, an isolated power system (IPS) is used. This system provides protection, power continuity, and alerts in case of equipment failure. The IPS is designed to minimize the shock hazard and does not require a power supply interruption in the event of a phase-to-ground fault condition. The isolation panel system is a small network with low leakage capacitance. It ensures that all branch circuits serving patient care areas are safe and protected. This includes the use of GFCIs and the implementation of an IPS for added safety measures.

provided with an effective ground-fault path by installation in a metal raceway system, or a cable having a metallic armor or sheath assembly.

• ELECTRICAL MEASUREMENTS 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 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 minimizing the probability of ventricular fibrillation to a factor as low as 0.002 (Limit of 10 μA for CF Applied Part under normal condition). 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 effect 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.

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,

<p>supply open &ndash; 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.</p> <p>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.</p> <ul> <li>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 and reversed.</li> <li>Enclosure Leakage, single fault, supply open - This test measures the enclosure leakage current with a single fault condition (Earth open). The current is measured through the Measuring Device with S1 open, S8 closed and S5 in normal and then S5 reversed.</li> <li>Enclosure Leakage, single fault, Earth open - This test measures the enclosure leakage current with a single fault condition</li> </ul>(Earth open). The current is measured through the Measuring Device with S1 closed, S8 open and S5 in normal and then S5 reversed. The Patient Leakage Current is the current flowing from the Applied Part via the patient to Earth or flowing from the patient via an Applied Part to Earth, which originates from an unintended voltage appearing on an external source.

• Patient Leakage, normal condition - This test measures the Patient Leakage Current under normal conditions. The current is measured through the Measuring Device with S1 and S8 closed, S5 normal and then S5 reversed.
• Patient Leakage, single fault, supply open - This test measures the Patient Leakage Current with a single fault condition (supply open). The current is measured through the Measuring Device with S1 open, S8 closed and S5 normal and then S5 reversed.
• Patient Leakage, single fault, Earth open - This test measures the Patient Leakage Current with a single fault condition (Earth open). The current is measured

through the Measuring Device with S1 closed, S8 open and S5 normal and then S5 reversed. The Patient Leakage F-Type Test (also known as mains on Applied Parts test) displays the current that would flow if a mains potential was applied to the Applied Part which was attached to a patient (i.e. a single fault condition). This test is applied only to type BF and CF equipment. The current is measured through the Measuring Device with S1 and S8 closed. S5 and S9 are switched between normal and reversed. The Patient Auxiliary Current displays the leakage current that would flow between Applied Parts under normal and fault conditions. For these tests, current is measured between a single part of the Applied Part and all other Applied Parts connected together. This test should be repeated until all combinations have been tested. This is also referred to as Applied Part to All. - Patient Auxiliary, normal condition - This test measures the patient auxiliary current under normal conditions. The current

is measured through the Measuring Device with S1 and S8 closed, S5 normal and then S5 reversed.

Patient Auxiliary, single fault, supply open - This test measures the patient auxiliary current under a single fault condition (supply open). The current is measured through the Measuring Device with S1 open, S8 closed and S5 normal and then S5 reversed.

Patient Auxiliary, single fault, Earth open - This test measures the patient auxiliary current under a single fault condition (Earth open). The current is measured through the Measuring Device with S1 closed, S8 open and S5 normal and then S5 reversed.

ELECTROMAGNETIC SAFETY

ELECTROMAGNETIC SIMULATORS

Electromagnetic simulation is a modern technology for simulating electromagnetic devices. It is more and more used to replace costly prototyping. Electromagnetic simulation software packages may be divided into two larger groups - circuit simulators and field simulators. Software for electromagnetic field simulation may also be