Keeping sophisticated medical equipment working properly presents a major challenge for today’s high-tech health care facilities.
Power quality problems can cause equipment failures at health care facilities in a heartbeat. When that equipment is sophisticated diagnostic medical imaging equipment like computerized tomography (CT), magnetic resonance imaging (MRI), ultrasound, mammography, and general X-ray equipment, the costly results include downtime, repairs, and replacement. Making matters worse, downtime exacerbates human suffering because it wreaks havoc on patient care delivery schedules, appointments, and confidence in test results.
To gain a better understanding of this issue, let’s examine some of the most common power quality problems found in health care facilities across the country. Over the last five years, GE Medical Systems (GEMS) power quality team auditors performed hundreds of proactive pre-installation and reactive audits to determine the causes of these power quality events. In most instances, improper grounding connections, unusual transformer wiring configurations, and loose connections were to blame.
Grounding. In 1996, GEMS put a program in place requiring pre-installation power and grounding audits for all new MRI, CT, and high-end cardiac and vascular X-ray systems. Not long after this program began, data trends showed certain problems were common to a significant percentage of sites. Most were typically easy to address with good wiring practices (NEC Art. 300) and compliance with basic grounding standards (NEC Art. 250).
At many audit locations, the GEMS team found that the branch circuit feeding the imaging equipment was missing ground conductors. Sometimes a conduit ground was the only ground return path found. And if there was a grounding conductor available, it typically came from a different grounding point in the facility—somewhere other than the source of the supply conductors.
At many older locations, the facilities managers didn’t even know where the earth-grounding electrode (ground rod) was located because it had been so long since it had been installed. This led many electricians to drive an individual grounding electrode for each new piece of imaging equipment. In other instances, the ground had never been connected to the system in the first place. The team even encountered situations in which the architect, engineer, or facilities manager recommended that an insolated ground (IG) be used when connecting new diagnostic imaging equipment. Although they had recommended an IG to improve reliability, more often than not, it creates more problems than it solves. For example, you can end up with two interconnected pieces of equipment without the same ground reference plane.
Such grounding issues will cause the following types of problems for medical imaging equipment:
Randomly appearing, intermittent software errors.
System lock-ups that require technicians to reset or reboot the equipment.
Random imaging artifacts that come and go in intensity and frequency.
Interaction with or interference from unrelated imaging systems in the facility.
Transformers. Transformers are another common problem area GEMS teams find when auditing the installation of medical imaging equipment. For example, some facilities do not have a 480V source readily available for the diagnostic imaging equipment they wish to install. They do, however, have access to a 208/120V power supply. Rather than special order a 208V delta to 480/277V wye transformer, some will install a 480V delta to 208/120V wye step-down transformer by reversing the connections because they can purchase these transformers as an off-the-shelf item at most electrical supply houses. However, this type of installation can burn up phase, neutral, and ground conductors during a fault condition.
How does this happen? The neutral on the primary side of the transformer provides a path for the fault current. This current in the primary windings of the transformer sets up a current in the secondary windings of the transformer, which in turn tries to drive the load. Then these secondary circulating currents try to “pull up” the primary phase that is down, and the neutral current rises too high. You can avoid this problem by eliminating the neutral connection on the primary winding of the transformer.
Here are a few more precautions to consider:
Do not use three transformers to create a wye-wye transformer bank. Because the transformers don’t share the same iron core, the third harmonic circulating magnetic fields do not cancel out, creating pseudo voltage of more than 300V and damaging any system or piece of equipment connected to it.
Do not connect the primary and secondary neutrals of an isolation transformer. This negates the benefits of installing such a transformer in the first place.
Do not use an ungrounded delta transformer to power diagnostic imaging equipment. This configuration can actually act like a voltage doubler. In this type of installation, GEMS teams have documented phase-to-ground voltage measurements as high as 960V on a 480V system. To eliminate this problem, use a wye or corner-grounded delta transformer to power all diagnostic imaging equipment.
Loose connections. You can find loose connections almost anywhere you look. In some instances connections are so loose that when touched, the conductors will fall out of the terminals. Check all terminals, fittings, and connections for tightness when performing a power quality audit. Just tightening connections and terminals can resolve a significant number of power quality problems.
Conclusion. Many of the power quality problems associated with the installation of diagnostic imaging equipment appear to be repeated in all parts of the country. These problems can generally be identified by a qualified electrical worker and are inexpensive to fix. To prevent thousands of dollars in new equipment from going up in smoke, it’s best to find these problems before equip- ment is powered up or turned over to the customer. Creating a list of common problems and sharing them with the people who will be working on these types of systems will go a long way toward avoiding power quality issues in the future.
Rush is the power quality team leader for General Electric Medical Systems, Belton, Mo.