Overview

The first two questions regarding EMI and EMC are: what is the difference between EMI and EMC? And which standards apply? Subsequent questions relate to how equipment can be made to meet the standards. Of course, meeting the standards often costs money (filter components, screening and suppressors) so the aim is to just meet the standards with a small safety margin.

EMI is electro-magnetic interference. This is the amount of radiation emitted by some equipment when it is operating. EMI is caused by emissions in the radio spectrum, which not only interfere with radio systems but also can cause other equipment to malfunction. One example is interference from portable radio transmitters like CB radios and cell phones; when used near a gasoline station, the pump can indicate the wrong amount being delivered. An often seen warning notice at a gasoline station says using a radio transmitter can cause a fire , but in reality the most likely effect is to cause an error in the fuel measurement. I did hear a story that CB radio users could reset the fuel counters by an appropriately timed transmission, but maybe this was just wishful thinking!

So what is EMC? This is electro-magnetic compatibility, and is a measure of how good a system is at rejecting interference from others. Medical systems have a high immunity requirement, because the consequences of a failure are death or injury. Any system connected to the AC mains power line must be immune to transient surges; the degree...