Why do we use cartridge fuses and not MCBs?

[JosephandAmber]

A customer asked me this today and I couldn't answer. Why, in LARGE switched fuses (60-100amp) do manufacturers use cartridge fuses and not MCBs?

 

[Julie.]

1) fuses are better than mcbs
2) we don't, there are mccbs and so on that provide similar protection to mcbs in larger sizes.
3) switch fuses are cheaper than mccbs

 

[davesparks]

It wouldn't be a switchfuse if it didn't have fuses in it.
Do you mean why would someone choose to use a switchfuse instead of an MCB or MCCB?
Fuses generally are better for protecting distribution circuits, they generally can clear higher fault currents and may discriminate better.
Fuses also, generally, allow a higher Zs.

 

[pc1966]

While it seems a simple question, the answers are very complicated.

Fuses have several advantaged over MCB/MCCB:

• Much lower maximum let-through energy
• Easier to achieve selectivity
• Lower one-off cost
• Generally higher maximum break capacity (e.g. BS88 fuses are often around 50kA or more, most MCB around 6-15kA, and few MCCB at/above 50kA)

But of course MCB/MCCB have other advantages:

• Lower let-through energy at medium fault currents (in the ~1kA PFC range where you have just entered the "instantaneous" trip region)
• Can be reset by non-skilled people
• Can be used safely to break/make under load
• No risk of accidental replacement with wrong value

Cascading MCB leads to poor selectivity: basically the upstream breaker's magnetic trip is often already well underway before the downstream MCB's contacts have opened enough. MCCB, at least the fancy expensive electronic types, can have a bit of a delay to achieve good selectivity with downstream MCBs trip characteristics.

Fuses, due to their smooth trip current/time curves, etc, find it easier to achieve selective with around a 2:1 rating difference (e.g. a 16A fuse down from a 32A fuse will always blow first) and even with the let-through behaviour of MCBs, still achieve acceptable selectivity for most combinations of MCB & PFC value.

So the TL;DR answer is: Fuses are far better, for a given cost, as an upstream OCPD choice.

If you have a spare half hour you might want to watch this semi-sales video:

View: https://www.youtube.com/watch?v=3dckmSgp1nw

[automerge]1596581555[/automerge]

Fuses also, generally, allow a higher Zs.

I forgot to mention that, please don't take my "Fuse appreciation society" badge away"!

Basically as you get to fault currents that blow a fuse in under tens of seconds the fusing time decreases very rapidly with increasing PFC. So for a given disconnection time requirement (whether 5s or 0.4s) you will generally find a fuse can meet it at a higher Zs (i.e. lower PFC) than a MCB, even though the MCB will trip quicker than the fuse when looking at lower overload levels that take minutes to clear.

 

[dmxtothemax]

Part of the reason we use HRC type fuses in industrial / large loads is
because HRC fuses ere designed to contain the sometimes learge arc that can occur when a large and / oir reactive load is suddenly isolated.
Something a standard type mcb would not handle.
They do make some industrial size circuit breakers with biult in ceramic lined arc shutes.
these types of breakers will handle it, not cheap mind !