EMC Question of the Week: December 15, 2025
Circuits operating at voltages greater than 50 volts and circuits whose conductors may come into contact with humans must generally have isolated
- safety grounds
- EMC grounds
- current returns
- all of the above
Answer
The best answer is “c.” For safety reasons, any conductor carrying a return current backed by a voltage greater than about 50 volts must generally be isolated from any conductor that could readily come into contact with a person. This is meant to ensure that exposed metal does not take on an unsafe voltage in the event that the current return path is broken. The transformer in the figure isolates a high-voltage AC current return (Neutral) from a low-voltage DC current-return path. Both sides reference the same chassis ground.
Safety grounds must not be completely isolated. Safety grounds ensure that exposed metal in different parts of a system or building have the same potential. They are not allowed to serve as current returns for power or signals with a voltage greater than 50 volts. They are designed to carry current only in the event of a fault. They should not be isolated from one another, because connecting them is the best way to ensure that they do not have an unsafe voltage difference.
EMC grounds have nothing to do with preventing electrocution, but they have a lot in common with safety grounds. EMC ground conductors cannot generally carry intentional high-frequency power or signal currents. Like safety grounds, they provide a local zero-volt reference. Like safety grounds they are the local destination for "fault" currents (i.e. the currents injected during BCI, EFT or surge testing). For safety reasons, EMC grounds are sometimes isolated at low-frequencies (kHz and lower). However, in components and systems that operate at high frequencies, it is often necessary to establish a high-frequency bond between any EMC grounds in close proximity.
Note that when conductors connected to safety ground carry intentional low-voltage low-frequency return currents, it is sometimes necessary to isolate these conductors to prevent unwanted common-impedance coupling. This is commonly referred to as "breaking a ground loop," although "ground loop" is a bit of a misnomer. Ground loops can only be a problem if one of the conductors in the loop is an intentional current-return conductor. For safety reasons, these "isolated grounds" cannot be fully isolated. They must still have at least one place where they are connected.
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