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The fuse from my microwave blew out last week. I removed the fuse and measured it with a multimeter to confirm the problem (It was measuring as infinite resistance)

The fuse cap reads the following: 15A/250VP. At the beginning of this string there is an F symbol encased in an L with a full stop at the end:

enter image description here

From what I understand the F is to show that its a fast acting fuse but I have no idea what the L under it means. I am also unsure what the P at the end of the string (250 VP) means.

The fuse seems to be ceramic and measures 31mm x 6mm. Searching for a replacement online I found this:

https://www.amazon.co.uk/gp/product/B085LGXZRZ/ref=ppx_yo_dt_b_asin_image_o01_s00?ie=UTF8&th=1

Would somebody be able to confirm if this is a suitable replacement? I'm mostly concerned about safety as i'm assuming that a wrong fuse can cause a serious problem down the line.

Any help is much appreciated.

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    \$\begingroup\$ The proposed replacement fuses look OK to me. Do realize that such a fuse doesn't just blow by itself. It needs more than 15 A to blow so that means there is/was some serious fault inside the microwave. If this fault isn't solved then a new fuse should blow immediately and your microwave might need repair or to be replaced. \$\endgroup\$
    – Bimpelrekkie
    Apr 29, 2020 at 8:59
  • \$\begingroup\$ The microwave originally turned on but I was having an issue with the mechanism used to action a microswitch when opening and closing the microwave door (the spring came out from the plastic holder). The fuse blew up because I tested one of the microswitches for the door with a screwdriver while plugged in (in hindsight extremely stupid). There was a quick spark, the fuse blew and the mains turned off. \$\endgroup\$
    – user2284355
    Apr 29, 2020 at 9:24
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    \$\begingroup\$ In that case you know what caused the fuse to blow and the new fuse should not blow. Do realize that the door switch in a microwave isn't something you should mess with when the microwave is powered as circumventing the switch means the RF power will "escape" and harm you when the magnetron tube is powered. Only work on the door switch with the mains unplugged. Use a multimeter to test the switch. Make sure the switch operates as it should before using the microwave again. \$\endgroup\$
    – Bimpelrekkie
    Apr 29, 2020 at 9:49
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    \$\begingroup\$ Realize that microwave ovens are very dangerous as they can emit high power RF waves and also contain high voltage DC (a couple of kV and that's buffered by a capacitor). If you can't explain the function of each component then I would advise against opening and repairing a microwave oven. \$\endgroup\$
    – Bimpelrekkie
    Apr 29, 2020 at 9:53
  • \$\begingroup\$ Hi @Bimpelrekkie many thanks for your help and particularly for the safety concerns you raised. I removed the microswitches and tested them with my multimeter in continuity mode. The multimeter beeps when the switch is activated and doesn't when the switch is off. The third terminal only activates when the switches are off. I have replaced the fuse with the one linked above and the microwave turns on again. The light turns on and off when the door is moved and the start light is available only when the door is closed. \$\endgroup\$
    – user2284355
    Apr 29, 2020 at 12:20

1 Answer 1

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Be aware that there are typically multiple micro-switches in a microwave oven that are designed to have a special operating sequence that prevents the magnetron from getting energized when the sequence is violated. The operating sequence of these switches is interconnected mechanically with the door open and latch mechanism. One very common way that the lockout works is that if the micro-switch sequence is violated one switch is wired across the AC power inputs in such a way as to force the input protection fuse to blow out.

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  • \$\begingroup\$ Thanks, as per the comments above this seems like a very plausible explanation to why the fuse blew in the first place. While the microswitches test out ok and the replacement fuse seems to have done the trick, i'm wondering if this could have damaged other parts that may become a danger when I actually try and heat something \$\endgroup\$
    – user2284355
    Apr 29, 2020 at 12:39

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