Replace a 4A SLOW BLOW FUSE with 4A MCB TYPE C

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Hi guys,

here is the case, normally we use a 4A SLOW BLOW FUSE to protect our 12V 8A Switch Mode Power Supply.

Whenever there is a short circuit the fuse blew before SMPS Short Circuit Protection Mode is triggered.

But if a 4A MCB TYPE C is replaced whenever there is a short circuit the SMPS Short Circuit Protection Mode will be triggered and MCB does not trip.

So I wonder it wont trip is it becuase it is a Type C MCB? For me it looks like the SMPS protection respond faster than Type C MCB. Any idea people?

Thanks in advance.
 

I check these components in web and.............

Look these graphics , you find out that

4A SLOW BLOW FUSE
2 x I time = 12s (fast blow 4s)
5 x I time = 3s (fast blow 0.02s)
10 x I time = 0.4s
20 x I time = 0.06s

MCB TYPE C (Resistance = 0.11 ohm , tripping power 1.8W )
2 x I time = 10s........
5 x I time = 1.5s........
10 x I time = 0.02s
20 x I time = 0.01s or less

but when I < 5 x I MCB acts in Thermal tripping area, timing tolerances are wide.

so if real shorted current is smaller than 5xI , MCB can be very slow

And surely there is some differencies in manufacturers data and tolerances


**broken link removed**

Look: Page 52

**broken link removed**

Look: Page 2

Regards
KAK
 

You don't mention if the trip is on the AC inout side (assuming it is AC) or the DC output side of the supply.
This introduces another variable, the MCB rating is probably only for AC, the fuse will be about the same rating on AC or DC. Check before using!

Brian.
 

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