Highfields Amateur Radio Club
Constructors Corner.
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De-coding Glass Fuses.
By Brian, MW0GKX.

If you want to know why you need a fuse, or why to replace with the correct one, see the Fuses, What and Why page.
For more information on other markings found on fuses see the Fuses: The Markings page.

I've had a few enquiries about coloured bands on fuses and what value is denoted by the colours etc. also what the meaning of various letter/number combinations on circuit boards or labels mean relating to fuse replacement.

I've searched long and hard to asscertain exactly what all this means (I did know some, but I've learnt a lot myself) and came across spotted fuses as well! I hope this page answers all the questions I've been asked and more besides.

Equipment Markings.

Internal (and some "back panel") equipment fuses are often Anti Surge or Time Delay or even "Normal", but how to tell the difference? Well it's easy when you know how! Look at the fuse rating (on the fuse itself or on or near the holder, often directly on the circuit board or a label or stamping on the case. Does it say T2AL250V or similar? (may not have all the characters, T3.15A was marked on a stereo I used to have) Yes? Good! The first letter(s) (TT, T, M, F, FF) tell you what type of fuse it is:

The number and the A (may be mA) is the amp rating of the fuse. The L denotes that it is a low breaking capacity or glass fuse (H is High breaking capacity and is usually a ceramic package). 250V is the rated voltage of the fuse (not greatly important in low voltage equipment and may be omitted from the fuse holder / circuit board information, if it is present then be sure not to use a fuse of a lower voltage rating). Taking the example above a T2AL250V fuse is a 2 Amp, Time Delay glass fuse rated for 250 Volts. A marking of F315mA would be a 315 milliAmp Quick Acting or Anti Surge, fuse.

If there is no leading letter(s) then it is a "Normal" type and the manufacturer was saving a little money (or was too lazy? Discuss!).

What happens if there are no markings around the fuse holder and the only marking on the fuse is coloured spots or bands? You're in the right place, read on!

Spots Before Your Eyes?

Some glass fuses have 1 or 2 coloured spots on them, sometimes in conjunction with the stamped markings on the end cap, but often instead of them. Here is a table showing the colours and fuse rating, in ascending order of current.
fuse 1 spot

fuse 2 spots
SALMON-PINK 50 mA
BLACK 60 mA
GREY 100 mA
RED 150 mA
BROWN 250 mA
YELLOW 500 mA
GREEN 750 mA
BLUE 1 A
LIGHT BLUE 1.5 A
PURPLE 2 A
WHITE 3 A
BLACKWHITE 5 A
ORANGE 7 A
ORANGEBLACK 10 A
ORANGEGREY 12 A
ORANGEGREEN 15 A
ORANGEPURPLE 20 A
ORANGEWHITE 25 A

2 Bands? Twice The Trouble!

Some glass fuses have 2 coloured bands on them.
If one band closer to an end cap AND WIDER than the other band (top image, below), read the value from the middle column.
BOTH bands the same width? (lower image, below) Then value you need is in the right hand column.
When decoding these fuses the band nearest the end cap (the wide one if they're different widths) is the type of fuse, the other colour being the value. Not all values may be available in all types.

I will deal with the type band first:
fuse 2 stripeYELLOW is Very Fast Acting,
fuse 2 stripeRED is Fast Acting,
BLUE is Normal,
GREEN is Slow Blow.
BLACK is Very Slow Blow.

The other colours are the value, thus:
NarrowWide
RED 1 A 10 A
PURPLE 1.25 A 12.5 A
ORANGE 1.6 A 16 A
BLUE 2 A 20 A
YELLOW 2.5 A 25 A
BLACK 3.15 A ---
BROWN 4 A ---
WHITE 5 A ---
GREEN 6.3 A ---

3 Bands? Easy!

Some glass fuses have 3 coloured bands on them, one band being closer to an end cap and wider than the other bands. When decoding these fuses the wide band (nearest the end cap) is the type of fuse, the other colours being the value. Not all values may be available in all types.

As before I will deal with the type band first:
fuse 3 stripeYELLOW is Very Fast Acting,
RED is Fast Acting,
BLUE is Normal,
GREEN is Slow Blow.
BLACK is Very Slow Blow.

The other colour is value, thus:
REDRED 100 mA
PURPLEPURPLE 125 mA
ORANGEORANGE 160 mA
BLUEBLUE 200 mA
YELLOWYELLOW 250 mA
BLACKBLACK 315 mA
BROWNBROWN 400 mA
WHITEWHITE 500 mA
GREENGREEN 630 mA
GREYGREY 800 mA

4 Bands? Here's The Info.

Some glass fuses have 4 coloured bands on them, one band being wider than the other 3. When decoding these fuses the wide band should be positioned to the right, then read the colours left to right. Not all values may be available in all types.

As with the 3 band, I will deal with the wide band first. This is, as before:
fuse 4 stripeYELLOW is Very Fast Acting,
RED is Fast Acting,
BLUE is Normal,
GREEN is Slow Blow.
BLACK is Very Slow Blow.

The other 3 colours, reading from Left to Right (holding the Wide band to the right) are the value, thus:
BLACKBLACKBLACK 32 mA
BROWNBROWNBROWN 40 mA
WHITEWHITEWHITE 50 mA
GREENGREENGREEN 63 mA
GREYGREYGREY 80 mA
BROWNBLACKBROWN 100 mA
BROWNBLUEBROWN 160 mA
REDBLACKBROWN 200 mA
REDGREENBROWN 250 mA
ORANGEBROWNBROWN 315 mA
YELLOWBLACKBROWN 400 mA
GREENBLACKBROWN 500 mA
BLUEORANGEBROWN 630 mA
GREYBLACKBROWN 800 mA
BROWNBLACKRED 1 A
BROWNREDRED 1.25 A
BROWNBLUERED 1.6 A
REDBLACKRED 2 A
REDGREENRED 2.5 A
YELLOWBLACKRED 4 A
BLUEORANGERED 6.3 A

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