Fuse Rate

One uses British Standard 7671 with full knowledge of the cable size, type and run. The fuses are there to protect the cable, not the motor; it is the overload device's job to protect the motor.

If in doubt, consult a qualified electrician.

Fuse works on the principle of heating & melting. Thus, only the RMS current matters. I don't undertanf why you have calculated the peak current for fuse selection.

Thank you that s what i need to know.

I have another doubt in time delay fuses. Just as i have mentioned above for a current rating of 123A rms, should i give my fuse rating as 135% of the nominal value( 123A ) ie. 166A. Or should i give the fuse rating the same of 123A, since time delay fuses will have a definite time delay for any overloads.If i give time delay fuse of rating 166A, it not only exceeds the nominal value upto 166A, but also continues until the time delay of 166A is over.Please correct me if i m wrong.

British Standard 7671 - again. WTF does the wiring look like <sigh>?

Although it is remotely possible to find just the right size fuse to provide proper motor protection on small motors, it is a really bad idea and nearly impossible at the sizes you are dealing with here. Using fuses only provides no protection against single phasing and is a good way to lose a motor.

Read PWSlacks post(s) again. Even if you don't want to acknowledge the British Stds, the principle behind them is the same all over. Thermal overload relays protect the motor, fuses are for short circuit protection. Size the cables to feed the motor, size the fuses to protect the cable against short circuits as per YOUR local installation standards. You appear to be over thinking this.

Oh come on. Have you never heard of standard component sizing. Good luck finding a 166A fuse. I want you to have a better understanding what happens when a single 100 ampere fuse blows.

Consider the starting current(starting method used)and its duration for which the fuse should not blow.

It's all in BS7671....

Oh, knock it off. There are regional standards for wiring that proscribe the fuse size for the power distribution cables used. A three phase power distribution capable of providing more than 100 amperes continuously at any power distribution voltage is nowhere for an amateur to work, let alone to design.

This is a fabricated homework problem. Has anyone here seen any motor current specified by a manufacturer to three significant figures in such a convenient sequence? I'm waiting to hear that the measured current is actually 123.456 amperes RMS. Then the question will be will the motor overheat? There's also no useful conditionals to the sparse but overly precise information we've been given. Is this current the starting current rating, the locked rotor current rating, continuous current rating, overload current capacity or the running current rating? Similarly we have no idea what is the starting circuitry for this motor; amortisseur windings, starter circutry poles with a centrifugal switch, prime mover?

Then there's the quench problem that this mythical fuse will have to contend with, as the back EMF from this multiple decades of horsepower motor tries to maintain a current draw while the fuse explodes open. Frankly I'm not comfortable of a three phase motor being protected solely by three independant fuses.

If this isn't a homework question (which it looks like it is), you shouldn't be using fuses for this application due to the valid points mentioned above. What do your local standards say?

If this is what your homework question looks like then that helps to explain the lousy state of knowledge that some graduated engineers I have noticed seem to have when they are taught misleading or illegal electrical practices that they then try and replicate in the industry by not following mandatory electrical safety codes and practices.

Before MCB/MCCB were introduced motors/cables were protected by fuses of different types depending on the load characteristics. What we need is some technical literature from reputed fuse manufacturers and more details of the load.

Read your local electrical code.

(IIRC, the U.S. NEC allows fuses to be the next standard size up from 1.75 x FLC, and circuit breakers the next standard size up from 2.25 (2.5?) x FLC.)

According to fusology i have seen that voltage rating of the fuse should be greater than or equal the circuit voltage. My question is to how much extent does the voltage rating of the fuse can be used?

For eg: i need to use a 4A fuse for a circuit of 24Vdc, i have seen fuses of 4A above 24V such as 32V, 125V, 250V, 600V, 1000V etc. My question is upto how much extent should i take the voltage rating of the fuse? Since this is for electronic circuit application what type of fuse should i use( material), can anyone suggest me?

So long as the voltage rating of the fuse exceeds the maximum voltage in the circuit, it is acceptable.

Can ac fuses be applied in dc circuits. What is exactly the difference between ac and dc fuses?

None.

If you are in NEC world, use NEC table 430.52. Per this table, the non-time delay fuse is 300% of motor full load current, the time-delay fuse is 175% of motor full load current for squirrel case motors (other than Design B energy-efficient motor).

- MS