Purpose of fuses between CB and motor OL relay

[ChefTurnedSparky]

In a motor starter, what’s the purpose? Are they that much faster to break the circuit than a CB? Is it just standard practice to put them there?
What type and size would I need for a 20A circuit, 1 hp motor? 208, single phase.

Crew lead told me to take picture of the data sheets in the starter, the clips and send it to purchasing with the HP/ fla of the motor. But that’s no fun for anyone. I want to know HOW and WHY!

 

[petersonra]

post a picture of the overload and contactor with the part number and someone will be happy to look it up for you.

I would not say it is standard practice but it is not unusual to need to put fuses in the circuit to achieve the required SCCR.

There should be a label that shows the available short circuit current. post a picture of that too.

I am surprised they did not go over this kind of thing in your apprentice school. Its been a major issue for several decades now.

UL determines the type of fuse and its rating based on testing it performs. The manufacturer will have this information available for you as part of the instructions or some supplemental material.

Just as a for instance, this is a SCCR chart for an AB contactors. It tells you what combination of fuses or circuit breakers can be used with that particular contactor to achieve a specific desired SCCR.

SCCR

configurator.rockwellautomation.com

 

[ron]

I am guessing that the circuit breaker you see in the starter is not a regular circuit breaker, it is likely a motor circuit protector (MCP) which is short circuit protection only. The fuses may be needed for short circuit ratings of the MCP.

 

[ChefTurnedSparky]

post a picture of the overload and contactor with the part number and someone will be happy to look it up for you.

Like I said, I want to be able to do it myself.

I am guessing that the circuit breaker you see in the starter is not a regular circuit breaker, it is likely a motor circuit protector (MCP) which is short circuit protection only. The fuses may be needed for short circuit ratings of the MCP.

I mean the breaker in the panel. But looking over 430.5 C(3)(a), breakers must be adjustable, and part of the motor controller. So the breaker in the panel only protects the conductors to the motor controller?

There should be a label that shows the available short circuit current. post a picture of that too.

this??

I have two identical starters, motors are single phase 208, 1HP and 3/4HP. According to table 430.248, FLC would be 8.8 and 7.6. Then 430.52(C)(1) says 175% max for AC polyphase, dual element fuses. So start at 10 or 15A fuses, Type R (look like rejection clips below?)

 

[garbo]

While in Vo Tech electric shop many years ago the teacher asked me what would blow first a 20 amp non time delay fuse or a typical 15 amp residential circuit breaker. I set up a test station where I used three different breaker & fuse manufacturers. Was amazed when I shorted the circuit out the 20 amp fuse always blow. Fast forward to the 1980's while working at a big plant owned by a fortune 500 company they were too cheap to purchase a circuit tracer for the 6 man electric shop and very few of the 277 volt lighting circuits were labeled. We had what boss called 1960 bullet proof 277 & 480 volt circuit breakers. We would remove one of the 277 volt male plugs from a light then use a cord to short out the energised wire to a steel beam. Bullet proof breakers often took several 5 second sparking producing events before clearing. Did not matter if panel was 25 or 250' away, all appeared to take too long to clear a short.

 

[Jraef]

The fuses are required because that is the only way the manufacturer UL listed the starter assembly at anything more than 5,000A SCCR. It’s right there in that paragraph that starts with “DANGER…”. So what they will often do is use fuses that are over sized on purpose essentially just as current limiters just to get the SCCR, but they use a breaker at a lower rating upstream so that it will trip first and be resettable.

 

[garbo]

The fuses are required because that is the only way the manufacturer UL listed the starter assembly at anything more than 5,000A SCCR. It’s right there in that paragraph that starts with “DANGER…”. So what they will often do is use fuses that are over sized on purpose essentially just as current limiters just to get the SCCR, but they use a breaker at a lower rating upstream so that it will trip first and be resettable.

Before retiring it would make my blood pressure jump when I was installing a $10,000 VFD and some manufacturers had a label that its only good for 5,000 SCCR. After that always ordered drives with built in disconnect switch and line fuses. For motors running off starters liked to use 200 K SCCR fuses.

 

[tallgirl]

While in Vo Tech electric shop many years ago the teacher asked me what would blow first a 20 amp non time delay fuse or a typical 15 amp residential circuit breaker. I set up a test station where I used three different breaker & fuse manufacturers. Was amazed when I shorted the circuit out the 20 amp fuse always blow. Fast forward to the 1980's while working at a big plant owned by a fortune 500 company they were too cheap to purchase a circuit tracer for the 6 man electric shop and very few of the 277 volt lighting circuits were labeled. We had what boss called 1960 bullet proof 277 & 480 volt circuit breakers. We would remove one of the 277 volt male plugs from a light then use a cord to short out the energised wire to a steel beam. Bullet proof breakers often took several 5 second sparking producing events before clearing. Did not matter if panel was 25 or 250' away, all appeared to take too long to clear a short.

You can find information like this on the trip time curves for the devices. You can also take one of those fuses apart and you’ll likely see something like a spring attached to the piece of fuse wire that’s just waiting for that wire to become soft enough to separate, while the circuit breaker is likely relying on a magnetic field to due the work.

As for why a dead short to a piece of structural steal would take 5 seconds to clear a short, regardless of the distance to the panel, recall that the structural steel is bonded at the service, not at the panel. That’s an entirely different path than had you tried the same thing to the EGC.

 

[petersonra]

Before retiring it would make my blood pressure jump when I was installing a $10,000 VFD and some manufacturers had a label that its only good for 5,000 SCCR. After that always ordered drives with built in disconnect switch and line fuses. For motors running off starters liked to use 200 K SCCR fuses.

Since most of the time vfds were listed for use with class cc or j fuses to get the higher SCCR, you pretty much had to use 200kaic fuses.

A fair number of low cost vfds got the default SCCR rating because they were ul listed but untested for higher SCCR. Once the SCCR craze got into full swing, they eventually went and got tested for higher SCCR with fuses.

The latest rule from ul seems to be no ul listing unless you pay for the SCCR testing.