Selection of HRC fuse & MCB rating

The fuse and/or mcb is sized to protect the wire, the wire is sized to handle the load...

https://www.bluesea.com/resources/1437

Thanks.

In a particular circuit if we use 2.5 sq.mm PVC CU cable whose current capacity rating is more than 25A , then what should be

rating of FUSE or MCB ?

From the above link I read that there might still be a difference for the fuse size dependent on the length of the circuit.

But then your wire seems to have a higher load carrying capacity. I would still go with he above scheme to be on the safe side.

This is dictated by code....If in doubt use minimum rating....Breaker size would be 15 amps...

http://electrical-engineering-portal.com/electrical-thumb-rules-you-must-follow-part-1

https://www.scribd.com/doc/35662243/Cross-Reference-AWG-to-Metric-Size-With-Ampacity-of-Insulated-Conductors

What's the matter? Don't you subscribe to your local national electrical code, or something?

Hi!

No I am not getting suitable answer from local.

Kindly help me in getting the rating of fuse & MCB rating.

Please refer the chain message.

Regards

Then use somebody else's, then. If you adopt UK standards, then you could apply BS7671, for instance, and you wouldn't need to post here. Some people are trying to get some sleep, you know?

The MCB is there to prevent average current over a threshold value from over-heating motors and similar load devices. If heaters, then the load may not be induction much at all, and also consider DC voltage source to eliminate the effect of inductive load, unless the DC is chopped back to AC down path.

A fuse is for overall circuit protection from surge (short-circuit, or major ground fault) current. The fuse burns out so the wires and motor does not. Typically the breaker may trip before the fuse blows, but if MCB is "sticky" and failing, then fuse is last line of defense. I think you will want probably something more than a 10 A fuse if the circuit is designed for 10 A nominal DC current. 15 A or 20 A if no 15 A can be had.

The MCB should be at least 150% of circuit, so it does not trip all the time, then fail, having been worn out.

The above is only suggestions, and you should not follow this rule of thumb, unless local codes and laws permit such a consideration within your country's standards.

If you're saying a 10 amp fuse blows at 10 amp, that's not right. It blows at about 20 amp. See #8.

You are right, after further review, the call on the field is overturned, 1st and 10 for Codemaster, it was indeed a fumble.

Having said that, I am probably wrong about the breaker too.

The only thing left in my defense, I did make a disclaimer at the bottom of my post, referring the enquirer back to the local codes. Hell, put a five or six amp fuse in there, and see if it holds, then move up to the ten amp one later. As far as the breaker, I agree the enquirer needs specific information about draw of his circuit to compare against the time characteristic of the breaker.

Re MCBs, found this on wiki if it's any interest.

International Standards, IEC 60898-1 and European Standard EN 60898-1, define the rated current I_n of a circuit breaker for low voltage distribution applications as the maximum current that the breaker is designed to carry continuously (at an ambient air tempe

International Standards, IEC 60898-1 and European Standard EN 60898-1, define the rated current I_n of a circuit breaker for low voltage distribution applications as the maximum current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C). The commonly available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A,^[5] and 125 A (similar to the R10 Renard series, but using 6, 13, and 32 instead of 6.3, 12.5, and 31.5 – it includes the 13 A current limit of British BS 1363 sockets). The circuit breaker is labeled with the rated current in amperes, but excluding the unit symbol, A. Instead, the ampere figure is preceded by a letter, B, C, or D, which indicates the instantaneous tripping current — that is, the minimum value of current that causes the circuit breaker to trip without intentional time delay (i.e., in less than 100 ms), expressed in terms of I_n:

<...prevent average current over a threshold value from over-heating motors and similar load devices...>

Ahem, er, actually that's the job of the motor overload unit. The MCB or fuse is there to protect the circuit conductors. BS7671 gives full protocols for coordination of protection.

I have about 40 years work experience in this field. One common mistake done by most people is not considering "Temperature De-rating". I used to ask my US or Europe equipment supplier to consider this point as in India ambient temperature goes up 50'C outside and inside some panels up to 70'C.

The obvious answer for a fuse is a rating of 10 amp. Fuses are designed to blow at 2x rating, ie 100% overload. But why an HRC fuse? My understanding is these are used when the load is a motor, to handle the high starting current but still protect when running. What is the load?

On MCBs I have curves somewhere showing trip time vs current. You can find them on the web, if I remember right you can select an MCB from these.

Full protocols for breaker and fuse selection for circuit protection may be found within the pages of British Standard 7671. <sigh>

Homework?

Hi!

From the discussion it is evident that for a 10A DC resistive load fuse rating could be 15A to 20A.

For MCB rating it can be 10A ( for curve C) as it will operate at the zone of 10 times of current during short ckt.& otherwise due to over load it will also trip but more time is required for that .

Your suggestion please.

You haven't got it, have you? You use a fuse or a breaker to protect the wiring. The load looks after itself, though a separate overload device to protect a motor is a very good idea.

Now, what you are trying to do is reinvent the wheel. All you've got to do is follow your local electrical code and you're fine. Don't follow it, and you're on your own.

If you're on your own then you can't expect any backup from your fire insurance company or your professional indemnity insurance company. Remember, as an Engineer, your job is to retire without having killed or maimed anyone during your career. As you seem unaware of this, and unwilling to learn it, this is my stop, and I'm getting off. Cheerio.

Hi!

There is no local no global!

Only knowledge & process speak. If you see in earlier days all electrical equipment were too large for its over design which in todays scenario it is much smaller & compact.

In earlier days the weight & size of ceiling fan is not same as that of todays'!

How it has happened?

It is due to quality value engineering & it is accepted by all customer.

Hence, minimum cost & maximum efficiency should be the major criteria while designing anything.

It is not reinvention or related to any insurance company.

It is just a revisit on selection criteria.

Hope you agree with this logic.

Cheers!!

Better magnets?

Probably in India you should use smaller fuse value due to the heat. It is getting hot in here just talking about it. God bless you for living there, you deserve it!

Where do you get fuse rated 15 or 20A from? Have you read my #8? 15 or 20A fuse would run the load OK, but so would replacing the fuse with a length of copper wire!

I'm no expert, and don't know offhand how the appropriate type of MCB - B,C,D etc is selected. I would tell the supplier the maximum current and type of load, and let him recommend something.

LOL!