Electrostatic Discharge Interference in the Clinical Environment
Brief cold snaps or humidification disruptions can cause BSD problems
by Wallace R. Elliott, MS CCE and Gilbert Gianetti
The ever-increasing concentration of microprocessor-controlled clinical instrumentation elevates the need to control environmental electromagnetic (EM) noise. Disrupting signals transmitted over power lines can be effectively controlled by the installation of surge suppressors or uninterruptible power supplies. The suppression of interfering signals transmitted through space can be much more difficult to accomplish, however, as anyone who is wrestling with communication signal- or electrostatic unit (ESU)- generated interference can attest to. Electrostatic discharge (ESD)-generated signal produces another form of interfering noise that can be EM-field-transmitted and similarly difficult to avoid.
ESD is the transfer of static charge between bodies of different electrostatic potential, in proximity or through direct contact.1 The more common form of ESD in the clinical environment results from a discharge through an air gap; an example would be a user accumulating thousands of volts of potential by walking across certain floor surfaces and then reaching for a device. EM field generation resulting from such ESD is difficult to model because the field radiates from conductors acting as antennas on both sides of the spark. Even reproducibil-ity of test results is difficult to achieve because control of the many sparks influencing parameters (e.g., contact approach speed) is complicated.2
Resource:
NFPA 99: Standard for Health Care Facilities
The consequences of interference created by ESD are often similar to problems created by power-line noise. A major difference, however, is that devices that are not even connected to wall power are susceptible. The broad-spectrum radiofrequency signal created by an arc can result in electromagnetic field transmission of adequate power to be received on [sic](j.p circuit boards, leading to disruption of control signals. Apparatus that is not designed with adequate fail-safe protection can create unpredicted situations when ESD-influenced [sic][iP operation degrades or disrupts control signals.
Diagnosing ESD Interference
Proving that ESD resulted in the malfunction of a device can be very difficult. One those occasions when apparatus obviously begins to operate erratically, coincidental with perception of an arc (Figure 1), diagnosis may be easy. When no arc is seen, evidence of the cause can be impossible to produce. Rebooting an ESD-influenced computer found to be operating in an error mode often brings a return to normal operation. After rebooting, it may be that no evidence of the malfunction will remain. Where evidence does remain it is likely to be microscopic and undetectable with common troubleshooting tools.
So how is ESD identified as the culprit responsible for producing these phantom nightmares? This is the challenge faced by technicians and incident investigators as well as clinicians. The solution to this puzzle relies on connections with circumstantial evidence.
Inexplicable problems occurring during very cold weather, for instance, are sufficient to make investigators suspect ESD interference. Once suspicion exists, the device manufacturer can be the best source of information. It has been the experience of the authors, however, that ignorance on the part of manufacturers' representatives regarding such problems should not be interpreted as conclusive evidence against ESD interference. Where the consequence of the interference is not dire, verification of the cause is often possible by monitoring natural repeat occurrences. If consequences are too potentially dangerous, artificial means of testing an ESD interference hypothesis must be contrived.
ESD Consequences
Consequences of ESD can range from clinician discomfort or inconvenience to life-support disruption. Fortunately, ESD-generated malfunctions frequently occur in the presence of clinical staff during periods of access to system controls. Generation of the electrical charge is most commonly produced triboelectrically by an individual's having walked across certain floor surfaces with rubber-soled shoes. Contact with clinical apparatus after accumulating such charge may involve clinical staff, support service staff, family members of the patient, or even ambulating patients.
The literature3 commonly categorizes the consequences of ESD in three ways: transient errors, soft errors, and hard errors. Transient errors may produce temporary disruption of data or control signals but normal operation returns with no human interaction. Soft errors require a manual reset or microprocessor reboot to produce a return to normal operation. Hard errors produce destruction of electronic components.
The consequences of interference created are often similar to problems created by power-line noise.
Removing a device that is suspected of being susceptible to ESD interference from the environment in which problems have occurred makes incident simulation difficult Even though clinical environmental conditions vary with daily changes in outdoor temperature and humidity, approximation of conditions under which incidents have occurred will usually have greatest success at the site where problems have been observed. This is the case since contributing factors include electrical (ground and interconnected apparatus) as well as temperature and humidity influences. Portable static charge generators are a convenient diagnostic tool. The use of static charge generators eliminates the need to closely control temperature and humidity conditions during testing. Testing conditions designed for both the laboratory and post-installation environments defined by IEC 1000-4-24 help in standardizing methods for evaluating a device's general susceptibility to ESD. Adherence to such protocols may not be optimal during attempts to determine whether or not ESD is responsible for unexplained phenomena, however. Much can be determined regarding specific response to ESD within a unique electrical environment by using the portable generator to reproduce observed or hypothesized circumstances—care must be taken while performing discharge tests. A test protocol that involves increasing discharge voltage magnitude in small increments does not ensure absence of permanent damage. Even having a reputable third-party incident investigator or private laboratory carry out tests can not ensure nondestructive results.
Incidents of ESD Interference
Following are descriptions of three situations investigated by the authors where ESD was concluded to have produced malfunction of clinical devices. In each case the device influenced was a model that is widely used at present.
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References
1. International Electrotechnica! Commission. Electromagnetic compatibility for industrial process measurement and control equipment. International Standard IEC801-2, Second Edition. Geneva, Switzerland: International Electro technical Commission, 1991.
2. Martin R. Designing for compliance: immunity to ESD. Compliance Engineering. 1993; 2:13-21.
3. Dash G, Straus I. Testing for ESD Immunity. Compliance Engineering, 1995 Annual Reference Guide. Boxborough, MA: Compliance Engineering, 1995.B8-11.
4. International Electrotechnical Commission. Medical electrical equipment. International Standard IEC 601-2-31, First Edition. Geneva, Switzerland: International Electrotechoi-caS Commission, 1994.
