What fuses for motors?

[mbrooke]

You would typically be mistaken with this thought. For example, if the motor is not loaded substantially, the current in the remaining phases may not increase significantly.

In that case neither would heater catch it. One phase opens- if the remaining two phases pull more current they either cause the soldier to spin the wheel tripping the starter OR the fuse link melts on one phase dropping the motor.

Some 60 years ago the NEC started requiring running Overload protective devices in each phase, for this very reason.

[/QUOTE]

Correct, for open phase conditions outside the structure:

http://www.cooperindustries.com/cont...le-Phasing.pdf


I know I'm being a hard head, but in so far I can't find any case against fuses providing overload protection. To be frank with everyone, I've never even fully understood the purpose of heaters. I always wondered why they didn't just sell a standard molded case breaker with the thermal and magnetic trip slightly tweaked at the factory.

 

[mbrooke]

Certainly, but will result in larger numbers.

True, but it gives an acceptable number albeit worst case.

 

[Tony S]

I would never dream of relying on fuses for motor protection.

For a start it would be against BS7671 regulations. Secondly, the cost comparison between an overload relay and the cost of a rewind or a new motor.
Call it a one off payment insurance policy.

Fuses protect the cable, the overload relay protects the motor.

 

[Besoeker3]

Let me get this straight. An EE is advocating NOT using a motor starter on 3 phase 480 motors in excess of X HP. WOW is this "value engineering" gone way wrong?

First time you blow one fuse, single phase that motor and ruin it, your value engineering has cost me a lot of downtime, labor, and money.

Totally agree. We mostly used thermal overloads with single phasing protection. When I was "growing up" the practical side of my learning was in a linoleum factory. They had manual star-delta starters. The overloads had oil filled dash pots. The viscosity of the oil gave the delay that thermal elements do these days.

 

[kwired]

Can we assume infinite bus?

Well if the desired selection won't hold during startup you either need a different a different trip curve or something to limit current during starting.

I have found many cases where you can get by with lesser fuses or circuit breakers than what is recommended as starting current is limited by source and/or the circuit. Also found many cases where you may need to use all 175 or 250% (especially for breakers with no mag trip adjustment) of the general code rules for short circuit/ ground fault protection to allow starting - all depends on conditions of the installation.

 

[ptonsparky]

Well if the desired selection won't hold during startup you either need a different a different trip curve or something to limit current during starting.

I have found many cases where you can get by with lesser fuses or circuit breakers than what is recommended as starting current is limited by source and/or the circuit. Also found many cases where you may need to use all 175 or 250% (especially for breakers with no mag trip adjustment) of the general code rules for short circuit/ ground fault protection to allow starting - all depends on conditions of the installation.

So you are saying he could put it all together as he wants and if it works, good? If not, what the heck, try again with a new motor and different fuses.

 

[mbrooke]

I would never dream of relying on fuses for motor protection.

For a start it would be against BS7671 regulations. Secondly, the cost comparison between an overload relay and the cost of a rewind or a new motor.
Call it a one off payment insurance policy.

Fuses protect the cable, the overload relay protects the motor.

What passage of BS7671 does it violate?

 

[Russs57]

Mbrooke, please understand I am a maintenance engineer at a hospital. I'm not one to quote code. I'm not interested in the lowest possible first cost solution. I'm interested in a robust, bullet-proof, quick and easy to service system. What is dangerous for me is likely perfectly safe for others. So with that in mind I'll offer the following based of my limited knowledge of code and 40 years of actual experience. All this assumes 480/277/4W/3PH with 1.15 SF motors greater than 1 HP.

As I understand it conductors must be sized for 125% of FLC per NEC table. Short circuit/ground fault protection ranges from 150% to 300% of FLC. IIRC it is 300% for non-time delay fuses, 175% for dual element time delay fuses, and 250% for inverse time breaker. For a 1.15 SF motor overload protection is 125% of motor nameplate amps. You can round up to next standard size expect for overload where you have to round down. Of course there are exceptions.

All of this can lead to some rather weird looking installations. Things like # 10 THHN on a 60 amp breaker, which is fine because overload protection would be set at 21.9 amps. So IMHO you are going to have a hard time picking fuses that will stay within that 125% requirement of nameplate amps (remembering that you have to round down), and reliably handling starting current. Fuses aren't cheap so you don't want them blowing every time you get a power blink.

You mention a 200 HP motor. Lets see what we have. A quick look says nameplate amps of 225 times 1.25 = 281 amps for overload protection. You have to round down so no 300 amp fuses allowed. Bet you have a hard time finding anything other than a dual element 250 amp fuse. You really think that is going to start that motor? Normally that fuse would be 400 amps if you had an overload relay.

Now to what I consider dangerous. I won't mention times I have been shocked or things that have caught on fire. This following is a real experience.

I had a set of three medical vacuum pumps. IIRC each pump had a 3 phase, 480, 10 HP motor. Each motor had an IEC motor starter with overload relay. Motor starter cabinet was fed from one breaker in an emergency power panel and motor starter cabinet had one MCCB for short circuit protection. One motor shorted to ground, IEC motor starter was blown to pieces with some melted wiring, causing upstream main breaker to trip on ground fault. Next thing you know I have a whole lot of patients without life safety/critical/equipment power (division of service wasn't required back then) and people on beds/stretchers trapped in elevators. All because someone didn't think three 3 phases fuse blocks with current limiting fuses were worth the price. You can bet your bottom dollar your's truly installed a new cabinet with class RK1 fuses and NEMA rated motor starters with electronic overloads the next day. Never had a problem again ever though other motors shorted to ground over the decades.

Too bad that sometimes bad things have to happen before the bean counters understand first costs versus life cycle costs.

 

[kwired]

So you are saying he could put it all together as he wants and if it works, good? If not, what the heck, try again with a new motor and different fuses.

Certainly

Or use the more typical motor overload protection devices. Many times you have a contactor as a controller anyway, not too big of a deal to add motor overload that matches up to the contactor or buy it as a complete assembly.

 

[mbrooke]

Mbrooke, please understand I am a maintenance engineer at a hospital. I'm not one to quote code. I'm not interested in the lowest possible first cost solution. I'm interested in a robust, bullet-proof, quick and easy to service system. What is dangerous for me is likely perfectly safe for others. So with that in mind I'll offer the following based of my limited knowledge of code and 40 years of actual experience. All this assumes 480/277/4W/3PH with 1.15 SF motors greater than 1 HP.


As I understand it conductors must be sized for 125% of FLC per NEC table. Short circuit/ground fault protection ranges from 150% to 300% of FLC. IIRC it is 300% for non-time delay fuses, 175% for dual element time delay fuses, and 250% for inverse time breaker. For a 1.15 SF motor overload protection is 125% of motor nameplate amps. You can round up to next standard size expect for overload where you have to round down. Of course there are exceptions.

Yup, no disagreement.

All of this can lead to some rather weird looking installations. Things like # 10 THHN on a 60 amp breaker, which is fine because overload protection would be set at 21.9 amps. So IMHO you are going to have a hard time picking fuses that will stay within that 125% requirement of nameplate amps (remembering that you have to round down), and reliably handling starting current. Fuses aren't cheap so you don't want them blowing every time you get a power blink.

I understand starting current is the crux of the issue- however- several fuse manufacturers are confident that their time delay fuses will handle these inrush currents while simultaneously providing good overload protection.

You mention a 200 HP motor. Lets see what we have. A quick look says nameplate amps of 225 times 1.25 = 281 amps for overload protection. You have to round down so no 300 amp fuses allowed. Bet you have a hard time finding anything other than a dual element 250 amp fuse. You really think that is going to start that motor? Normally that fuse would be 400 amps if you had an overload relay.

Honestly, I have no idea- but two fuse manufacturers are confident that it will.

Now to what I consider dangerous. I won't mention times I have been shocked or things that have caught on fire. This following is a real experience.

I had a set of three medical vacuum pumps. IIRC each pump had a 3 phase, 480, 10 HP motor. Each motor had an IEC motor starter with overload relay. Motor starter cabinet was fed from one breaker in an emergency power panel and motor starter cabinet had one MCCB for short circuit protection. One motor shorted to ground, IEC motor starter was blown to pieces with some melted wiring, causing upstream main breaker to trip on ground fault. Next thing you know I have a whole lot of patients without life safety/critical/equipment power (division of service wasn't required back then) and people on beds/stretchers trapped in elevators. All because someone didn't think three 3 phases fuse blocks with current limiting fuses were worth the price. You can bet your bottom dollar your's truly installed a new cabinet with class RK1 fuses and NEMA rated motor starters with electronic overloads the next day. Never had a problem again ever though other motors shorted to ground over the decades.

Too bad that sometimes bad things have to happen before the bean counters understand first costs versus life cycle costs.

That sounds like a complete lack of selective coordination, and to be honest having individual short circuit protection for each motor does not fix your problem. What if there is a buss fault inside your motor control center or the fuse block melts down? In your case the MCCB and/or the branch circuit breaker should have tripped, not the whole riser or critical power system. Which to be honest is to common in critical application.

 

[Tony S]

What passage of BS7671 does it violate?

Since the girlfriend tidied the flat I haven’t a clue where my reg’s book or anything else is. Being retired I don’t really need them anymore.

I will find out and let you know.

 

[mbrooke]

Since the girlfriend tidied the flat I haven’t a clue where my reg’s book or anything else is. Being retired I don’t really need them anymore.

I will find out and let you know.

Thanks

 

[Russs57]

Mbrooke, have you looked at prices yet?

I'm getting the impression you would have a circuit breaker, a contactor, and a 3 phase fuse block with fuses (not to mention a safety switch/MCCB at the motor). If I am wrong correct me. If not I bet you will find, once you get past a certain HP, that you can buy a NEMA fusible combination starter cheaper than the separate parts you are advocating. You are going to need something within line of sight at the motor anyhow.

I may not of seen every post but I didn't see anyone in favor of not having an overload relay. Keep in mind we aren't trying to sell you anything, unlike a fuse manufacturer.

 

[mbrooke]

Mbrooke, have you looked at prices yet?

I'm getting the impression you would have a circuit breaker, a contactor, and a 3 phase fuse block with fuses (not to mention a safety switch/MCCB at the motor). If I am wrong correct me. If not I bet you will find, once you get past a certain HP, that you can buy a NEMA fusible combination starter cheaper than the separate parts you are advocating. You are going to need something within line of sight at the motor anyhow.

I may not of seen every post but I didn't see anyone in favor of not having an overload relay. Keep in mind we aren't trying to sell you anything, unlike a fuse manufacturer.

Good point- what would typically be the price difference?

Ideally I want circuit breakers, but I can't see typical molded case circuit breakers holding on inrush. At least many are saying that here and I have no reason to doubt them. Certainly not Jraef who eats, sleeps and breaths motors, VFDs and OCPDs. Huge respect toward him.


Regarding fuse manufacturers, don't ever hold back on criticism. If anyone knows anything and/or is willing to badmouth fuse manufacturers I'm all eyes and ears :thumbsup:

 

[mbrooke]

Just leaving this here, from another thread: