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#75049 - 02/12/07 01:05 PM Redundant motor protection
drillman Offline
Member

Registered: 03/24/03
Posts: 97
Loc: Somewhere in Texas
Allow me to describe my situation.

There is a 75 HP, 3 phase pump that has a combo NEMA size 4 starter.

The starter is fed with 1/0 from the main 480 V switchboard using a 3 pole 150 amp breaker.

The wires come into the starter and land on the top of a molded case switch that is cable operated at the handle.

At the bottom of the molded case switch, there are short pieces of 1/0 wire going to a fuse block.

The fuses are 600V, 150A, type RK5.

At the bottom of the fuse block the wires come out and loop up to the top of the starter.

Why do I need those fuses in the starter if the feed breaker is the same size? Seems to me like a case of double protection …… or is it?

A thermal scan shows a bad connection at one of the fuses and instead of replacing it I was thinking of just eliminating the fuse block and fuses.

Thanks for any help.

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#75050 - 02/12/07 11:44 PM Re: Redundant motor protection
frenchelectrican Offline

Member

Registered: 02/06/03
Posts: 938
Loc: Wi/ Paris France { France for ...
well for the situation you got there and you say molded case switch.

ok if that molded case switch have no OCPD in there then you MUST use the fuse to protect the starter.

the other reason why you need molded case switch is used for local disconnection usage

the fuse you have for the motor is correct size there

but i will like to double check the starting and running current if possible to make sure it is ok as well

Merci , Marc
_________________________
Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)


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#75051 - 02/13/07 06:29 AM Re: Redundant motor protection
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
You may be correct that you have redundant motor protection.

The upstream breaker may be sized correctly for short circuit and ground fault protection in which case the fuses are not needed. The motor starter may have the correct sized overcurrent/overload protection in which case the fuses are not needed.

But, the molded case switch may not have the correct stand-alone short circuit current rating (SCCR). This manufacturer of the switch shold be contacted to see if it can be protected by the upstream breaker rather than by the integral fuses.

[This message has been edited by JBD (edited 02-13-2007).]

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#75052 - 02/13/07 07:01 AM Re: Redundant motor protection
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
I advise against making the change.

First of all, not every "molded case" breaker is designed with motor protection in mind. While this is something that usually comes up in 'motor control centers,' rather than combination starters, it is something to consider.

More important, fuses respond much more quickly than any breaker; this is why some equipment specifies 'fuse,' rather than simply 'overcurrent protection.'

It is so tempting, so easy, to see a blown fuse as an inconvenience, to blame the fuse. Don't forget that when a fuse blows, it does so for a reason .... too many see their job as 'changing the fuse,' rather than fixing the problem that blew the fuse in the first place.

Review the application as well; many times the wrong fuse is installed.

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#75053 - 02/13/07 07:00 PM Re: Redundant motor protection
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Motor protection (i.e. HACR) has been a standard test for all UL489 multi-pole breakers for at least 10 years. Except when they are in their current limiting range very rarely are equally sized fuses "always" faster than breakers. In fact because of the speed that breakers operate is one reason that the NEC allows them to sometimes be sized at higher values than time-delay fuses.

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#75054 - 02/14/07 09:11 AM Re: Redundant motor protection
Helectric Offline
Member

Registered: 01/07/05
Posts: 91
Loc: Brandon, SD
Your post doesn't say if there is any other type of overload protective device being used here. Is there an overload relay present?

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#75055 - 02/14/07 05:53 PM Re: Redundant motor protection
drillman Offline
Member

Registered: 03/24/03
Posts: 97
Loc: Somewhere in Texas
Yes there is a contactor with an overload relay in the combo staterter box, its an across the line type. I actually replaced the contactor a few years ago as one of the contacts came apart.

I forgot to mention in my first post that this building and equipment is 20 years old. The building is a prison and the pump is used for condensor water for water source heat pumps. So it is needed all year long. The building is about 100,000 sq ft.

I want to thank all for the help. I never seriously consindered doing away with the fuses. I wanted to research options more than anything else. The other option was to buy a breaker that will replace the molded case switch but I am sure thats going to cost more than a fuse block.

I suppose I could figure out the SCCR rating of the upstream breaker, but thats going to take more time and money than just replacing the fuse block. Besides how do I explain that to the insurance company/jury after the prison burns down the next day after I modified the equipment??????

The fuse has not blown yet, but it is getting mighty warm and will blow sooner rather than later. To make matters worse, the backup pump does not work. The motor works fine but theres some sort of problem with the pump that the HVAC guys have not resolved yet..... The other problem is that the pump that works now has developed a leak and will need to be shut down to repair, but without a backup...... Around here things tend to go bad all at once.

The motor is drawing slightly less than FLA which is 96 amps or so. Its drawing around 90 amps per leg.

I should mention this is a 20 year old motor came with the building. About 2 years ago a bad bearing was discovered by the HVAC guy and it was sent off to a motor shop to be repaired and baked. It has been in use for 6 months and so far no problems.

Once again thanks for all help.

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#75056 - 02/14/07 06:17 PM Re: Redundant motor protection
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Just a little note for all those out there who might not be clear on the roles played by different parts of a motor circuit....

The motor has a nominal horsepower rating. We use this rating, together with the NEC, to size the wires feeding the motor.

The NEC also has tables for selecting the fuse or breaker size. We can use these, but we usually use the motor nameplate information.

Some equipment ... HVAC stuff comes to mind ... has nameplates that specify the maximum and minimum overcurent device sizes. Uf the nameplate reads "maximum fuse," then fuse you must use; breakers are considered inadequate by the manufacturer. Whether fuse or breaker, the size is usually quite a bit larger than any normal running load.

The fuses, or breakers, get power to the starter. They are there for "overcurrent" protection - that is, to protect against a serious, immediate fault, like a short.

A "starter" is nothing but a contactor with the addition of overLOAD protection, commonly called 'heaters.' These protect against long, sustained, over loading of the motor. These are often sized very close to the usual running amps of the motor. The newer electronic ones provide additional protections, against things like low voltage, single phasing, etc.

What is important is that you not confuse "over current" with "over load" protection. Each is distinct.

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