What is electrical safety?
Biomeds perform electrical safety on medical devices to ensure patients are protected from dangerous leakage and fault currents. Leakage currents exist in all medical devices. A fault current is a current that flows through any path other than to power the medical device. The ground wire of the power cord connects all exposed metal parts of the medical device to ground. The grounding resistance is kept below approximately 0.15 ohms (set by local regulations) and is usually checked during preventative maintenance, prior to first use of the medical device and after each repair. A patient’s skin contact resistance can vary between 60,000 ohms to a few thousand ohms. If the medical device contacts the great vessels, the patient’s resistance can drop below a few hundred ohms and offer a direct current path through the heart. Current levels between 16 and 80 milliamps that flow through a patient can cause tissue damage, respiratory or cardiac arrest! A properly grounded medical device will ensure most fault and leakage currents flow to ground, instead of the patient, if the patient is in electrical contact with the medical device.
What are leakage currents?
Leakage currents exist because no insulator has an infinite impedance and all equipment has stray capacitance. Proper grounding supplies a path for these leakage currents to ground. On medical devices that use catheters or electrodes to connect to the patient, these allow a low impedance path through the heart for current to flow. If a total resistance of 1 megaohm exists between the equipment case and hot line, a leakage current of 120 microamps would flow to ground through the ground wire. This could flow through a patient if the ground wire is at fault and the patient is in electrical contact with the medical device.
What is macroshock and microshock?
A macroshock is a current that flows through the patient when touching the medical device, the point of contact being the skin. The current flow through the body can take many different paths which may include the heart. Macrocurrents from medical devices are regulated around the world to about 500 microamps or less. A microshock is a current that flows through the patient via an invasive point such as a catheter or an electrode. An invasive point has a much lower resistance than typical skin contact. These currents also may flow through multiple paths through the body and are especially dangerous if they flow through the heart, which may cause ventricular fibrillation or micro-electrocution. Microcurrents from an invasive point are regulated around the world to about 100 microamps or less. Biomeds must know the local electrical safety regulations that apply to their medical devices.
What is a fault current – single fault condition?
In North America, most equipment is powered by 120 volts AC @ 60 Hz. Power is supplied using 3 wires which are colour coded black, white, and green. The black wire is referred to as the hot wire as it is 120 volts relative to ground or earth. The white wire is referred to as the neutral wire as it is 0 volts relative to ground or earth. The green wire (also may be a bare wire) is a ground wire, not intended as a main current path. It is intended to ensure a fault and/or leakage currents flow through it to ground, instead of through the patient. This power is accessible to equipment via a 3-prong plug inserted into a 3-prong receptacle. In a properly wired and grounded system, the hot and neutral wires supply 120 volts to the equipment and a circuit breaker limits the maximum current allowed by the load or medical device. The ground wire ties all exposed metal parts to ground potential or 0 volts. An example of a single fault condition could be the hot wire touching an exposed metal surface. Current will flow through the black wire to the chassis then to the green (bare) wire to ground. Since the current flows to ground instead of through the equipment, this is considered a fault. Assuming a ground resistance of 0.15 ohms, the amount of current would be 800 amps. This level of current would trip the circuit breaker until the issue is fixed. At the same time, 60 milliamps of current will flow through the person, if they are touching an exposed metal part, assuming a skin contact resistance of 2,000 ohms (skin contact resistance can vary between about 60,000 ohms to a few thousand ohms).
What is a fault current – double fault condition?
A double fault condition could be the hot wire touching an exposed metal surface (fault one as outlined above) and the exposed metal surface(s) not connected to ground (fault two). If a person touches an exposed metal part and the ground was open (infinite impedance), the current will flow through the person to ground, its only path. If the person’s contact resistance is 2,000 ohms, a current of 60 milliamps would flow through them, considered a fault current. A current of 60 milliamps could cause serious damage to a person or even death. This example emphasizes the reason a biomed ensures the ground connection to the medical device is intact and within local regulations, typically 0.15 ohms or less!
How does the Biomed ensure a medical device is electrically safe?
Electrical safety tests are performed by the biomed using a special testing device referred to as an electrical safety analyzer. The electrical safety analyzer must be capable of testing the locally regulated electrical safety standard. The analyzer’s purpose is to test all electrical testing scenarios which allow measuring leakage currents under varying conditions. The medical device’s power cord plugs into the electrical safety analyzer which supplies power to the device and allows opening the ground (among other tests) to simulate a fault, during leakage measurements. The analyzer also has connectors for attaching ECG patient leads and/or applied parts (such as a defibrillator’s paddles) to allow various leakage measurement scenarios. Electrical safety tests are performed based upon the local regulations but commonly performed on a preventive maintenance schedule, prior to using the new medical device and after each repair. Common electrical safety checks performed by the biomed include:
1) Chassis ground resistance: this is the ground resistance measured between a grounding point on the medical device’s chassis and the grounding prong of the electrical power plug. This is usually kept below 0.15 ohms but based upon the local regulations.
2) Chassis leakage: this is the leakage current that may flow through a patient if touching an exposed metal part of the medical device. Leakages are measured under a few different scenarios and must be below local regulations.
3) Patient leads leakages: this includes leakage currents that may flow through a patient from any ECG lead to ground and between leads. These currents are measured under a few different scenarios and must be below the local regulations.
4) Applied part leakages: this is the leakage currents that may flow through a patient from any conductive applied part such as a defibrillator’s paddle or patient monitor’s temperature probe. These currents are measured under a few different scenarios and must be below the local regulations.
What is an electrical safety standard?
Countries around the world recognize the dangers of electricity and how associated currents can affect the body. Most countries have regulations they have developed or adapted, which measure and regulate these currents. A standard developed by the International Electrotechnical Committee (IEC) is the IEC 60601 standard also referred to as the IEC 601 standard. It is a series of standards that regulates the safety of mechanical and electrical devices. It has evolved over the years since first published in 1977. Many countries have adapted the IEC 60601 standards such as in the USA (UL2601-1), Canada CSA C22.1, EC (EN 60601) and Australia/NZ (AS/NZ 3200-1). Other countries do not have a national standard but follow the manufacturer’s guidelines which are commonly the IEC 60601 standard. Biomeds ensure they are following the locally regulated electrical safety standards!
What is the IEC 60601 standard?
The IEC 60601 standard divides medical devices into three classifications which are B, BF and CF.
1. B stands for body contact between the patient and electromedical equipment excluding contact between the heart or great vessels. B type of equipment cannot exceed 100μa from chassis or applied parts under normal conditions. With a single fault the leakage current must not exceed 500μa.
2. BF stands for body contact floating (or isolated) between the patient and electromedical equipment excluding contact between the heart or great vessels. BF type of equipment cannot exceed 100μa from chassis or applied parts under normal conditions. With a single fault the leakage cannot exceed 500μa.
3. CF stands for cardiac contact floating (or isolated) between the patient and electromedical equipment. A CF type of equipment cannot exceed 100μa from chassis or 10μa from applied parts. With a single fault the leakage cannot exceed 500μa from the chassis or 50μa from the applied parts.
