Avoiding the Common Mistakes of Circuit Protection

Originally Published MDDI November 2003

Circuit Protection

Circuit protection is not difficult, but it is crucial to patient care.

Ken Cybart

It's only a circuit breaker—how hard can choosing it be? Yet many designers of medical equipment make mistakes when specifying their device's circuit protection. Because medical products are intimately connected to patient care, proper circuit protection is crucial. Discussed in this article are the most common errors in specifying such components.

Single-Pole Circuit Breakers

In medical equipment applications, the most common mistake is to specify a single-pole instead of a two-pole circuit breaker. As Figure 1 shows, two-pole circuit breakers have four terminals: two for the power side, two for the load side. If the voltage is 230 V or higher, two-pole circuit breakers should always be used. 

Even when the voltage is 115 V, it is safest to interrupt both the live wire and the neutral side to compensate for the possibility of incorrect wiring or crossed wire colors somewhere in the building. If the equipment is wired incorrectly, operators using a device with a single-pole circuit breaker 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.

Two-pole breakers are required by UL 544 and UL 60601, the two standards governing medical equipment and devices. You won't find a UL 544– or UL 60601–listed circuit breaker; the standards apply to the entire piece of medical equipment. However, most UL-recognized two-pole circuit breakers satisfy the requirements for use in UL 544 and UL 60601 equipment.

Protection-Level Standards

Another common error is specifying for incorrect environmental conditions. Terms such as drip-proof, water-splash protection, and dust-proof are commonly used, but may be misleading unless standard definitions are applied. 

When specifying, use established standards as a measure, such as international standard EN 60529/IEC 529, which defines the degrees of protection for electrical equipment. Using this standard, designers can decide which protection is correct for their particular application. 
For example, a combination switch and circuit breaker might need water-splash protection if the device will be wiped down with disinfectant; it would not need a rating for immersion in water. Typically, circuit protection devices used in medical equipment should be rated IP 54, which, according to EN 60529/IEC 529, means dust- and water-splash-protected. Truly watertight and dusttight circuit breakers are available, but are expensive and often unnecessary.

Specifying Current Rating

Concerned about nuisance tripping, many engineers specify circuit breakers with higher ratings than needed. Overspecifying the current rating is caused by confusion between fuses and circuit breakers. Engineers are accustomed to oversizing fuses as a way to prevent nuisance tripping, but there is no need to oversize a thermal or thermal-magnetic circuit breaker.

Unlike a fuse rating, a circuit breaker rating indicates the maximum current that the circuit breaker will consistently maintain in ambient room temperature. Thus, a 10-A circuit breaker will maintain a 10-A current without nuisance tripping. Also, 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. For example, a typical 4-A circuit breaker with a slow trip profile will tolerate a temporary 10-A current surge without nuisance tripping.

Nuisance tripping is often caused by inrush currents associated with certain electrical components found on motorized equipment, such as x-ray equipment, MRI machines, and hospital beds and tables. In addition to motors, components such as transformers, solenoids, and large capacitors also create high currents upon start-up. In such cases, the designer needs to specify a circuit breaker that has 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.

Fuse or Circuit Breaker?

Fuses may be appropriate for nonvital medical equipment. They provide inexpensive circuit protection, but the cost savings should be weighed against the low total cost of ownership of circuit breakers. 

Foremost, circuit breakers can be reset quickly, correcting and restoring the circuit with minimal downtime. Also, there is no assurance that a replacement fuse will be properly rated. 

If a fuse is replaced by a higher-rated fuse, overheating and equipment failure may occur. Circuit breaker trip-characteristic curves are stable over time, but as fuses age, their trip characteristics change. This may lead to nuisance tripping and increased downtime.

Circuit breakers also offer designers more options than do fuses. An auxiliary contact may be added to a circuit breaker that can communicate an alarm condition to an LED indicator or process software. Remote trip is another option available with circuit breakers but not fuses.

Unlike fuses, circuit breakers have a variety of types and trip profiles, and therefore can be more precisely matched to loads and environment.
In applications requiring all live current–carrying wires to be positively opened from the source voltage when a fault occurs, fuses cannot be ganged together to assure that all lines will be opened in the event of an overload or fault. And finally, since fuses cannot be tested without destroying them, how can the designer be sure the fuse in the equipment will open if there is a critical overload condition?

Circuit Breakers as On/Off Switches

Many circuit breakers are designed to be both a circuit breaker and on/off switch (see Figure 2). For medical equipment used in critical patient-care applications, a combination device provides increased reliability by decreasing the number of component connections. One fewer installed component provides one less reason for the equipment to malfunction. 

Other benefits are decreased consumption of panel space, reduced wiring and cost, and increased protection over ordinary switches. All UL 1077–recognized circuit breakers that function as switches must pass an endurance test of 6000 on-and-off cycles at rated current.
When a standard (non-circuit-protective) switch is used, it is often backed by a fuse. Fuses should not be used in equipment destined for operating rooms and other applications where time-consuming fuse replacement can be life-threatening. 

Matching the Circuit Breaker to the Application

There are four basic design choices in circuit breaker technology: magnetic, thermal, thermal-magnetic, and high performance. Each has a different trip profile in relation to time and current, and each has distinct and different mechanical characteristics (see Figure 3).

Magnetic Circuit Breakers operate via solenoids, and trip nearly instantly when the threshold current is reached. This type is appropriate for printed circuit boards and impulse disconnection in control applications. Often, a magnetic circuit breaker is combined with a hydraulic delay to make it tolerant of temporary current surges. 

The circuit breaker should be mounted in a vertical plane to prevent gravity from influencing the movement of the solenoid. If the circuit breaker is mounted in a horizontal plane, derating is required. 

Thermal Circuit Breakers incorporate a heat-responsive bimetal strip or disk. This type has a slower characteristic curve that discriminates between safe temporary surges and prolonged overloads. Thermal circuit protection is appropriate for machinery or equipment where high current inrushes accompany the start of electric motors, transformers, and solenoids.

Thermal-Magnetic Circuit Breakers combine the benefits of a thermal and magnetic circuit breaker: a delay that avoids nuisance tripping caused by normal inrush and surge currents, and fast response at higher currents. High overcurrents cause the solenoid to trigger the release mechanism rapidly, while the thermal mechanism responds to prolonged low-value overloads. These breakers have a two-step trip profile that provides fast short-circuit protection of expensive electrical systems while minimizing the risk of disrupted system operation from temporary surges and normal overloads.

High-Performance Circuit Breakers provide high interrupting capacity and excellent environmental specifications where reliable operation under rigorous conditions is required. These circuit breakers are typically designed for aerospace, defense, and similar heavy-duty applications where extreme vibration, mechanical shock, and other conditions are present, and where circuit breaker performance is absolutely critical. 

For some high-performance applications, thermal circuit breakers are available with a compensating element that eliminates sensitivity to ambient temperature. Consider using high-performance circuit breakers in applications such as medical equipment used aboard an emergency ambulance.

Many engineers seek specification assistance from the support desks of circuit breaker manufacturers. However, it is important to be wary of advice from manufacturers who make only one type of circuit breaker. The vendor's application engineers should recommend the right type of circuit breaker for the specific application, regardless of their own technology.

Selecting Correct Actuation

There are many types of actuators, including press-to-reset, push-pull, push-push, rocker, toggle, and press-to-reset with manual release. The actuator type is more than a cosmetic consideration. Critical medical applications, such as equipment used in an operating room, usually call for push-pull actuators, because they are the most resistant to accidental actuation. Another good option is a rocker switch with a plastic guard.
The type of actuator a designer selects should be determined by the location of the circuit breaker, the need for illumination, the need for human operator safety or convenience, and the consequences of accidental engagement.

Specifying Interrupting Capacity

Interrupting capacity is the maximum amperage a circuit breaker can safely interrupt. Circuit breaker manufacturers publish this specification along with the number of times the circuit breaker will perform this feat. 

One type of interrupting capacity specification is called Icn, or normal interrupting capacity. Icn is the highest current the circuit breaker can interrupt repeatedly (three times minimum, per IEC 934 and EN 60934 PC2). This specification indicates the circuit breaker's ability to withstand certain overloads and continue to operate after the fault is corrected. 

Another type of specification is UL 1077 interrupting capacity. UL 1077 (or IEC 934 and EN 60934 PC1) is the maximum current a circuit breaker can safely interrupt at least one time without causing a fire hazard.

To comply with various standards, engineers must specify circuit breakers with adequate interrupting capacity. Unfortunately, applying the appropriate standard may be confusing.

For example, UL 489 molded-case circuit breakers provide interrupting capacity of 5000 A and higher. While perfectly appropriate for main power distribution applications, this standard has been perpetuated in many industries where the available short-circuit current, governed by circuit resistance, is much lower. 

The UL 1077 standard for supplementary protectors covers the short-circuit test and lists the maximum available fault current at which the breaker was tested. Although certain devices such as UL 489 molded-case circuit breakers have higher interrupting capacities, they may not be well suited for lower-current applications, where precise overload protection and adequate short-circuit protection are better provided by a UL 1077 supplementary protector.

High interrupting capacity is not always necessary, especially for circuits with dc motors, because low-voltage dc circuits are far more susceptible to resistance. In most low-voltage dc applications, a UL 1077–recognized circuit breaker with 2000-A interrupting capacity is more than adequate.

Nevertheless, patient-connected equipment is a different matter. Higher risks associated with direct patient care may make it desirable to specify a UL 1077 circuit breaker that also has a high interrupting capacity.

Matching the Type of Terminal to the Device

Circuit breakers with plug-in-style quick-connect terminals simplify installation and replacement, and they may also be soldered. Quick-connect terminals may be used for circuit breakers rated up to 25 A. 

However, medical equipment that needs highly reliable connections, such as incubators, will benefit from circuit breakers with screw-terminal connections. Screw terminals are also advised for high-vibration equipment, such as laboratory centrifuges and shaker tables. Stud terminals and other special terminals can be provided for high-current and OEM applications.

Providing Necessary Spacing in the Design

It is important to maintain recommended minimum spacing requirements between non-temperature-compensated thermal circuit breakers. A mere 1 mm spacing between breakers is all that is required. Without this tiny thermal gap, the circuit breakers can heat up and increase the sensitivity of the bimetal trip mechanism. If the breakers must touch each other, they should be derated to 80% of their normal amperage rating.

Conclusion

While circuit protection is not difficult, it is important. Following these tips will help designers of medical equipment avoid common mistakes that could hurt both performance and patient safety.  

Copyright ©2003 Medical Device & Diagnostic Industry