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I am a medical physicist, and I program software for MRI scanners as well as develop electronic devices that interact with them. Because of the way MRI works, it is necessary to use very large power amplifiers (~300 kVA) to drive large currents through low resistance coils of wire, producing a precisely controlled magnetic field that varies linearly in space (and is in addition to that produced by the main, static magnetic field). These devices are called "gradient sets", and there are three of them, each with an associated power amplifier. The load is almost entirely reactive.

I recently had cause to look at the wiring of one of these amplifiers into the wall. Its isolator is a large three phase supply mechanical switch, a photo of part of which is below. There are two electrical symbols on the box for a switch -- one with a square on it and a 160 A rating, and one without.

What do these symbols mean? Not being an electrical engineer, I've never seen them before, and some quick googling hasn't shown anything up. Is one stating that there is a 160 A RMS fuse present (at three phase 415 V), but that the instantaneous current might be up to 315 A?

The symbol in question

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The symbols imply Fuse switch disconnector and switch disconnector as shown in figure.

enter image description here

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    \$\begingroup\$ Does that mean that there is a fuse in the switch, and it will blow at 160 A, and that there is a risk of damage if the switch is used when currents above 315 A are flowing? \$\endgroup\$ – Landak Mar 3 '16 at 16:29
  • \$\begingroup\$ The fused switch will blow above 160 amps (approximately... fuses aren't the most precise things.) The 315 A rating of the other switch is irrelevant. \$\endgroup\$ – Li-aung Yip Mar 5 '16 at 8:05
  • \$\begingroup\$ The rating (315 A) rating next to the switch symbol makes sense only this way: It tells you to use a switch that is capable of switching currents of at least 315 A. \$\endgroup\$ – zebonaut Jun 21 '16 at 9:07

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