Short Ckt Current and Protection for Induction Motor

If the fault current is only 57 amps, your motor will not start if it requires 462 amps. I would suggest you make that fault current calculations again.

Indeed. That would suggest her wiring is far too small.

Sounds like schoolwork.

I would advise against using an MCB, better by far to use a Motor-Protection CB as these are designed for the application.

Examples that I have personally used include GE's Surion, Schneider's TeSys, and Allen-Bradley's 140-series.

Here's a pic of the GE version - they all look much the same :

the FLC is 14.43 Amps,

using DoL starter, starting current is: 115.47 Amps,

with 0.25 pu subtransient reactance, the fault current is: 461.89 Amps.

Regs,

"Fault" Current? The 462 A will flow through its INTENDED path. That is not the definition of a "fault" current. Fault current is what flows through an UNINTENDED path. Only if some part of the winding is shorted, THEN there will be a "fault" current. Perhaps i am nitpicking.

Anyways, 460A is hardly something to worry about, since it is so short in duration. Damage is caused by I²t.

The best thing you can do is to use MCCB ir fuses for short-circuit protection, a contactor for switching the motor on and off and an overload relay for overload and single-phasing protection. Economical and sensible. An MPCB may be able to replace the MCCB+Overload relay, but i am partial to the thermal overload relay which is somewhat more accurate than the MPCB.

Your call.

I politely disagree about separatethermal overload relay being more accurate than the thermal release of a specialized motor circuit breaker. It doesn't matter, what counts is that the correct class is chosen.

The case is different if you use advanced electronic motor protection relays as those can offer more protection functions than thermal relays but they're also way more expensive (up to USD 20'000 or more for very large MV motors) and are typically only used with larger motors. (Note that I'm not referring to basic electronic motor overload protection relays which are merely a replacement for conventional motor overload protection relays, their advantage is a lower thermal drift and a lower power dissipation but they've also some drawbacks.)

When using a motor overload protection relay, the feeding circuit breaker does not need to feature a thermal release (overload function), a short-circuit protection (magnetic release) is sufficient as an overload protection of the wiring between the breaker and the thermal relay is not required (at least as per IEC, I don't know the U.S. and Canadian codes at all).

Personally I prefer all-in-one motor protection breakers like the one from the posted picture (just as example, not referring to any specific brand), unless automatic reset of the overload protection (which I'd only recommend in special cases) is required they're fine.

Of course you are entitled to your opinion. As i am to mine. The class only specifies one point on the characteristic if i recall. Many other features are not particularly elaborated in the standards. i have been involved in the design of a few relays and also an MPCB. My opinion stems from the trade-offs we designers had to do.

I should have been more precise, indeed you're right when referring to some manufacturers.

Personally I'm not much worried because I mainly use products from either major manufacturers (e.g. ABB, Siemens,...) or less known highly specialized manufacturers, mostly from Germany and its surrounding countries as well as Norway, Sweden, Italy or so. Some North American products are great too but in low voltage there aren't that many products available for IEC low voltage markets (excepted for example Allen Bradley). Some Japanese products wouldn't be bad as such but they tend to lack support in some countries.

There also lots of exotic lower cost manufacturers but I don't know them much. It's not that they're necessarily all bad but here for most projects clients wouldn't accept them anyway.

Thank you, i worked for ABB and spent considerable time with the designers of the thermal relay T25DU at Heidelberg. Prior to that, i worked with Eaton at their UK (Bedford) operations. After retiring, i helped a licencee of Schneider here in India. Among other things, on the Telemecanique relay. So, pardon me, i am a little biased towards the thermal relay

We have been using English Electric make HRC fuse for short Circuit protection, Siemens make thermal relay for overload protection for our hoist panel. This is specified in Indian Standards and it worked well for years.

Having worked for some years for English Electric, i am happy to read your complimentary remarks. They were the best, non-aging, absolutely guaranteed performance. i am proiud of having worked for some top notch fuse designers, some have written books on this. One was knighted for his contribution to electrical technology. i agree that the fuse+ contactor+thermal relay has proved reliable under Indian conditions for years.Does well in Type 2 coordination too as i have said elsewhere.

Thanks for your information.

I've nothing against thermal relays. Conventional versions have been around since a long time and do a great job. Electronic versions are more recent but overall they seem to work well. Users mostly check costs and if there are some specific requirements like due to a unusually wide operating temperature range, space, automatic reset, space or so. Motor protection circuit breakers are simple to install and there's no risk of pilot wire wiring error as they always switch off all phases while with thermal relays you've to wire the coil of the contactor or a breaker undervoltage release. They're also fully protected against short-circuits up to the rated specs while in some cases thermal relays don't show a sufficient withstanding level.

Now when it comes to reliability, as detailed reliable MTBF information is not much available most who don't want to take risks simply choose device from major manufacturers but without analyzing much in detail the reliability of each used device. Overall I don't worry about thermal and other motor protections devices, experience shows that there are way more issues for example with 24 VDC power supplies (even some series from top brands are now made in China and I had reliability issues with them).

This fault current doesn't include fault current contributed by other motors working simultaneously.What is the fault level at the motor terminals?.

The device you show is for motor over current protection only.

The circuit protection for a motor is 125 % of full load. The circuit breaker is designed for up to short circuit protection. You do not need to add the short circuit calculations for circuit breaker size.

Not so. The device shown provides both short-circuit protection and adjustable overload protection. A variant that offers short-circuit protection only (mag-trip) is also available.

The device shown is Not for short circuit protection. Fuses or a circuit breaker has to be involved. This is straight from the NEC.

Duly noted

From Airpax

British Standard 7671 gives full protocols for motor circuit protection under the sub-heading of Co-ordination.

In my opinion, you change the start current with short-circuit current.

Subtransient impedance of an induction motor it is the ratio Irated/Istart.

It is very high [0.25].I think 0.15 could be most understandably.

Since the rated current could be 14.25 A then 57 A it could be your start [or locked rotor]-current.

The short circuit current also could be 462 A.

A transformer of 100 kVA 4% zk and a supply cable of 3*2.5 sqr.mm copper of 670 m will present an impedance [approx.] of 0.5 ohm and the short-circuit current at motor terminals will be 400/sqrt(3)/0.5=462 A.

This sounds like the same guy with the (18) dam gates. Once upon a time, the motors were 17.5 kW; now they are 8 kW. And still no relevant info, but lots of irrelevant fluff.

For 8 kW unless using a crappy motor protection circuit breaker you're typically in the 50 to 100 kA range so why worrying? In my typicals there is one 100 kA breaker family up to about any motor current up to about 20 A (3x400 V IEC), always contactor coordination type 2 anyway. So up to 100 kA it's a straightforward process to choose the breaker, I've just to check the busbar short circuit ratings.

Of course mileages may vary, especially if costs require you (I mean it generally, not the poster above) to use the cheapest crap avaiblable.

A vaporware dam? :)

Especially as in many case hydraulic cylinders are the better option for dam gates. Forget all those gears and winches wherever possible.

Sometimes I wonder if some users just ask for asking or if there are really parts of the world where totally inexperienced people are involved in technical decisions in larger projects (maybe the answer would be politically incorrect). And yes, if you travel enough you'll sooner or later find GNYE conductors used as phase. Don't even waste time asking, don't believe blindly in replies, always check yourself...

In my opinion if the short-circuit current is actually 462 A, a MCCB of 18 A [1.25*Irated] will trip at 462 A in 0.04 sec. The conductor temperature [40 oC ambient from 90 oC rated] will not be more than 120 oC.

The minimum short-circuit rated will be ok [6-10 kA rated].