EMI Risk Assessment in a Hospital Ward With One and Two Roaming Wireless Transmitters

To control electromagnetic interference, hospitals often specify that mobile transmitters may not be brought any closer than a minimum separation distance (MSD) to an electronic medical device. This paper investigates the risk that the field strength due to mobile transmitters exceeds the immunity level of a medical device. The spatial variation of the field strength can be characterized by the well-known Ricean probability distribution, using the Sabine method to evaluate the parameters. The mobility of transmitters is accounted for by assuming a function for the probability that a transmitter is present at each location throughout the hospital room. The risk of exceeding immunity is estimated with no restriction on the movement of mobile transmitters, and the reduction in risk is estimated when an MSD policy is enforced. Staff may not fully comply with the MSD, so the increase in risk with specified levels of non-compliance is found. It is shown that with some non-compliance the risk of exceeding immunity becomes constant with increasing MSD, and so specifying a larger MSD does not necessarily increase safety.