Line Isolation Monitor (LIM)

The past Line Isolation Monitors (LIM's) are used in hospital operating rooms and intensive-care units to continuously monitor their isolated, ungrounded power systems. A perfectly isolated, ungrounded power system presents no shock hazard to patients or medical personal. However, the isolation isn't perfect and parasitic current paths exist. These current paths may allow leakage currents to flow. The Line Isolation Monitor continuously calculates the maximum leakage current (hazard current) that may flow from the power system to ground. This leakage current may flow through a person's body and present a shock hazard to that person. Agency standards specify a maximum allowed hazard current that might flow in a isolated, ungrounded power system. The LIM displays the calculated level of the hazard current and sounds an alarm if the hazard current exceeds a pre-determined level. The standards require that in-service LIM's be manually tested on a regular basis to insure that it is operational and that it is periodically checked for proper calibration. Square D's LIM's are sold world-wide and need to operate from numerous power systems. These include 100V, 120V, 208V, or 240V and 50 Hz or 60Hz and single phase or three phase. The prior analog technology LIM had to be built and inventoried as sixteen unique products to accommodate all of the combinations of line voltage, frequency, and phase. And, it required a skilled technician to calibrate, both at the factory and at the hospital. This interactive calibration procedure required up to twenty minutes of time to complete. The future The Elsyn developed and patented digital LIM uses innovative digital signal processing techniques to calculate the hazard current. This digital LIM is built and inventoried as a single product. The end-user sets a few switches on the LIM circuit board to configure it for his unique installation. At power-up, this LIM automatically calibrates and tests itself in about 2 minutes. Also, the LIM continuously calibrates and tests itself during normal operation. Agency standards are being revised to acknowledge the features of the digital LIM. In-service digital LIM's will not need to be tested and checked as often as the old analog LIM's. The digital signal processing algorithms allow two LIM's to operate on the same power system. This is a customer requested feature that could not be met with the analog LIM. Summary Elsyn's involvement in this project began with a casual conversation with the marketing manager for the LIM. In this conversation he indicated that he would like to have a LIM that was less expensive to build, easier to calibrate, less expensive to inventory, and less expensive to maintain, than his existing product. Through our own initiative we developed a concept for the digital LIM, developed a proof-of-concept model, and proposed a development project to turn our concept into a product. Square D contracted with Elsyn to develop our digital LIM. The result of the development project is a product that meets all these objectives, is in production and being sold world-wide, and has been awarded three U. S. Patents! Elsyn's commitment to the LIM development project has given Square D a product that is technically superior to their competition's product.

Home | Design Equilibrium | Results | Team | Design Tips | Contact Us | Capabilities | Patents |