The Ultimate Designer’s Guide to Overcurrent Protection in Medical Devices: Safeguarding Lives and Technology

Medical devices play a crucial role in the treatment and diagnosis of illness and disease. The IEC 60601-1 standard, “Medical Electrical Equipment—Part 1: General Requirements for Safety,” is the foundation document addressing many risks associated with electro-medical products defined as “equipment, provided with not more than one connection to a particular supply mains and intended to diagnose, treat, or monitor the patient under medical supervision, and which makes physical or electrical contact with the patient and/or transfers energy to or from the patient.”

This includes devices such as EEG monitors, IV pumps, imaging systems, ECG machines, vital signs monitors, and other diagnostic and therapeutic equipment that connect directly to patients.

Devices without direct patient interaction are classified as laboratory test equipment and are typically governed by the IEC-61010 standard. This standard, titled "Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use," outlines essential safety guidelines for such equipment.

Circuit Protection in Medical Devices

Circuit protection devices fulfill two primary purposes – safety and reliability. Safety is assured by disconnecting power in a circuit during overcurrent, which protects against electrocution and fire hazards. In addition, circuit protection may be necessary to comply with government or industry standards.

Reliability is a fundamental requirement for electrical medical equipment, ensuring seamless performance in doctors’ offices, operating rooms, and laboratories. It plays a pivotal role in equipment design and is a top priority for manufacturers, who must uphold strict quality control standards within the medical industry.

According to the Underwriters Laboratories (UL), a circuit breaker is “a device designed to open and close a circuit by manual means, and to open a circuit automatically on a pre-determined overcurrent, without damage to itself when properly applied within its rating.” Circuit breakers come in various sizes and mounting configurations for use on racks, panels and printed circuit boards, as well as specialized installations for high-voltage electrical use.

Each device has different time characteristics and must be used and applied according to the manufacturer’s recommendations for the individual application. For instance, the trip time of thermal circuit breakers depends on the overload current. The higher the current, the faster the breaker will reach its predetermined tripping temperature.

What is Overcurrent?

Current and voltage together supply the power we use every day. Any electric current that exceeds the rating of the circuit is an overcurrent. Overcurrents can be divided into two categories: overloads and short circuits.

Electrical circuits are designed to handle a specific amount of electricity. An overload occurs when you draw more electricity than a circuit can safely handle. Without a circuit breaker, an overload would cause the circuit wiring to overheat, which could melt the wire insulation and lead to a thermal event that can severely injure personnel and compromise or destroy equipment.

A short circuit is any current not confined to the normal path. The term comes from the fact that such currents bypass the normal load (i.e., it finds a “short” path around the load). Accidental contact usually causes short circuits or worn insulation, and is more serious than overloads. Damage occurs almost instantly. Examples of short circuits include two or more conductors accidentally touching, someone touching or dropping tools across energized conductors, or accidental connection between energized conductors and ground.

Exceeding the rated load for the circuit wiring causes the circuit breaker to trip, shutting off the power to the entire circuit. The Short Circuit Current Rating (SCCR) represents the maximum level of short-circuit current that a component or assembly can withstand when protected by a specific overcurrent protective device. A common misconception of SCCR is that the interrupting capacity or rating of a circuit protection device is also the SCCR of the end use equipment in which it is installed. It is not. The Interrupt Rating, instead, is the highest current, at rated voltage, that a device is intended to interrupt under standard test conditions.

Circuit Breakers vs. Fuses

A quick word about circuit breakers and fuses: compared to fuses, circuit breakers can quickly and reliably be reset after tripping, restoring the circuit with minimal downtime. In addition, there is no guarantee that a replacement fuse has the proper rating. If a higher rated fuse replaces a fuse, over- heating and catastrophic equipment failure may occur. Circuit breaker performance is relatively stable over time, but as fuses age, their trip characteristics change – not usually for the better.

Fuses should also not be specified in medical equipment located in operating rooms and other applications where time-consuming fuse replacement can be life-threatening.

Common Errors

Reliability is critical in medical equipment, and false alarms must be prevented. Many engineers concerned about nuisance tripping specify circuit breakers capable of carrying rated currents continuously. Engineers are accustomed to oversizing fuses to prevent nuisance tripping, but there is no need to oversize thermal or thermal-magnetic circuit breakers.

?A product which, through either incorrect selection or poor design, is prone to nuisance operation and results in an increase in equipment downtime. Conversely, a protection device that fails to operate as required is likely to have dire consequences, including fire hazards, and product liability implications. A correctly specified circuit breaker will minimize or eliminate these risks.

Inrush currents often cause nuisance tripping associated with certain electrical components found on motorized medical equipment, such as MRI machines, x-ray equipment and hospital beds. Motors, especially those that start under load, can draw several times their normal current when starting. Medical equipment can have a high inrush load for a very short duration (0.1 msec). In addition to motors, components such as transformers, solenoids, and large capacitors create high currents upon start-up. Power converters also often have inrush currents much higher than their steady state currents, due to the charging current of the input capacitance.

The difficulty of providing overcurrent protection is exacerbated by the need to accommodate these temporary current surges. A typical thermal or thermal-magnetic circuit breaker can withstand inrush currents up to 2.5 times its rating or higher for a limited time without tripping. The overcurrent protection device must react quickly to overload or short-circuit faults, but must not interrupt the circuit when typically harmless inrush current flows.

The designer needs to specify a circuit breaker with an adequate delay. Thermal circuit breakers have a natural delay, and magnetic circuit breakers are available with added hydraulic delays. The specified delay should match the duration of the expected inrush or temporary surge currents.

Mistakes can occur when specifying the degree and types of protection. Terms such as water splash and dust proof are common under Ingress Protection of components, while other standards such as DIN EN 60529/IEC 529 define degree of protection of electrical equipment. Using these standards will help engineers decide which protection is appropriate for their application.

In medical equipment applications, a common misstep is to specify a single pole instead of a two-pole circuit breaker. Two-pole circuit breakers have four terminals: two for the power side, two for the load side. If the voltage is 230V or higher, two-pole circuit breakers should be used.

If wired incorrectly, staff members operating the equipment protected by single-pole circuit breakers are at risk of electric shock when it trips. Two-pole circuit breakers provide a much safer solution, as they interrupt both wires from the source of power. It’s also important for both phases of a 240V line to be switched off simultaneously, or you risk severe damage to the load.

Another common error is failing to provide spacing in a design for circuit protection. As a rule, it is important to maintain the recommended minimum spacing requirements between non-temperature compensated thermal circuit breakers. Only a 1mm space is generally required, but without this thermal gap, the circuit breakers can heat up and increase the sensitivity of the trip mechanism.

Don’t overlook the fact that many circuit breakers are designed for multi-functional use. Reliability can increase with one component, providing both overcurrent circuit protection and on / off switching in equipment. Why? The two-way circuit breaker decreases the number of component connections. Fewer installed components provide less opportunity for failure or equipment malfunction, reduces the amount of thru holes for mounting and wiring of components, while providing a most compact design. All UL1077 recognized circuit breakers that act as switches must pass an endurance test of 6000 on-and-off cycles at rated current.

Supplementary and Branch Circuit Protectors

Circuit type is important in determining what type of circuit protection device can be utilized. There are basically three main circuit types: feeder, branch and control. The National Electrical Code defines such circuits as the portion of the wiring system between the load side of the overcurrent protection device or the power-limited supply and all connected equipment. Applying circuit breakers and supplementary protectors depends on the circuit type.

Circuit breakers are required for:

• Feeder Circuits – All circuit conductors between the load side of the service equipment and the line side of the final branch circuit overcurrent device are feeder conductors. The goal of feeder circuit protection is to localize and isolate the fault, short circuit or overcurrent from the remainder of the service.
• Branch Circuits - These are the circuit conductors installed between the final overcurrent device protecting the circuit and the outlet. The circuit primary (line side) requires branch circuit protection. In a hospital, for instance, medical diagnostic equipment will be on the equipment branch circuit, and a “critical branch” circuit may be required in areas where emergency power must be available for patient care. Circuit breakers are mounted in panel boards to protect branch circuit wiring. The applicable standard is UL489. The requirements of this standard cover molded case circuit breakers, circuit breaker and ground-fault circuit-interrupters. The general rule is that a UL 489 circuit breaker can simultaneously protect multiple devices, such as an entire control panel. UL 489 branch circuit breakers are designed to serve as the primary overcurrent and short-circuit upstream circuit protection of an electronic panel. Supplementary protectors are a circuit breaker type device that can be used for both primary (line side) and secondary (load side) protection. This type of circuit is downstream of a branch circuit device. As the name suggests, supplementary protectors supplement the circuit protection already in place. The applicable standard is UL 1077 - “Standard for Supplementary Protectors for Use in Electrical Equipment”.

Proper Placement is Essential

As part of the system design process, the engineering team specifies the required circuit breakers and where a current interruption device should be placed. This requires planning. In every case, the upstream breaker (with short-time delay) should be greater than the ampacity of the downstream one to achieve coordination between circuit breakers and assure the downstream feeder breaker has enough time to “clear” before the fault condition pushes the upstream breaker into its trip curve.

Summary

Circuit breakers reliably disconnect hazardous overcurrents. They help increase the availability and life of medical equipment, and they also reliably protect doctors, nurses and patients against the hazards of overheated equipment or electrical shock.

Each circuit breaker type has a characteristic curve of overcurrent vs. time to trip, an interrupting capacity, dynamic characteristics such as what happens during the circuit opening process, and other characteristics that may be design and type-unique.

If the circuit protection system is not well designed and correctly selected, electrical equipment operating in the patient vicinity, even though operating perfectly, could still be hazardous to the patient. A piece of equipment that causes a short circuit or power overload can trigger a protective system upstream and shut down other possible life sustaining equipment. The goal is to specify circuit protection that is “just right.”

To ensure safe, reliable performance of medical devices, designers must factor in circuit protection requirements from the earliest stages of the circuit design process.