Choosing the Right Fuse

[goodoboy]

Hello,

Which fuse to select, RK1 or RK5?

I want to select 15 amp fuse, but whats the difference between Rk1 and Rk2?

15amps is the min circuit amps for a/c unit. RLA is 49.50amps. 480V three phase.

My wired is sized for 15amps

 

[GoldDigger]

Hello,

Which fuse to select, RK1 or RK5?

I want to select 15 amp fuse, but whats the difference between Rk1 and Rk2?

15amps is the min circuit amps for a/c unit. RLA is 49.50amps. 480V three phase.

My wired is sized for 15amps

It appears (see http://www.omega.com/Auto/pdf/ecnr_ecsr_lenrk_lesrk.pdf, quoted below) that the only difference between the two types is that the RK1 limits the fault current to a lower value, which could reduce calculated Arc Flash incident energy and help avoid over stressing other breakers which are in series. Unless you have some really exceptional conditions, I doubt it will matter for you.

The application recommended for these fuses [RK1] is exactly the same as for the Edison ECNR/ECSR fuses [RK5] except for the advantages of greater current limitation.
The Edison LENRK/LESRK fuses have up to 40% more current limitation and up to 350% more Amps-Squared-Second (I2t) limitation under fault conditions than Edison ECNR/ECSR fuses to reduce potential for damage.
In addition, LENRK/LESRK fuses allow better selectivity for electrical power system designers and better short circuit protection for breakers having inadequate interrupting ratings.
ECNR/ECSR and LENRK/LESRK fuse lines are physically interchangeable (and electrically interchangeable per U.L. equipment listing conditions)....

 

[goodoboy]

It appears (see http://www.omega.com/Auto/pdf/ecnr_ecsr_lenrk_lesrk.pdf, quoted below) that the only difference between the two types is that the RK1 limits the fault current to a lower value, which could reduce calculated Arc Flash incident energy and help avoid over stressing other breakers which are in series. Unless you have some really exceptional conditions, I doubt it will matter for you.

Thanks.

I don't have any exception, I just can't tell the difference for my application.

I am thinking of going with RK5 because http://www.cooperindustries.com/con...-datasheets-b/Bus_Ele_DS_1001_LPS-RK_0-60.pdf :
Provides motor overload, ground fault and short-circuiprotection. When used in circuits subject to surge currents such as those caused by motors, transformers and other inductive components, these fuses can be sized close to

 

[StarCat]

Fusing 3 phase HVAC gear

Fusing 3 phase HVAC gear

Hello,

Which fuse to select, RK1 or RK5?

I want to select 15 amp fuse, but whats the difference between Rk1 and Rk2?

15amps is the min circuit amps for a/c unit. RLA is 49.50amps. 480V three phase.

My wired is sized for 15amps

3 PH HVACR machines if at all possible should never be fused but protected with HACR type breakers.
If you arrive at a fault that takes out one fuse but the compressor is not compromised, it will then in most cases be very quickly killed by the resulting single phase condition, where as a circuit breaker will " often " allow the attending Tech to correct the lower level fault before the most expensive part of the machine is destroyed. I have seen this countless times and the commercial trend in the state I came from was definitely away from using fused disconnects on that type of gear.

 

[kwired]

3 PH HVACR machines if at all possible should never be fused but protected with HACR type breakers.
If you arrive at a fault that takes out one fuse but the compressor is not compromised, it will then in most cases be very quickly killed by the resulting single phase condition, where as a circuit breaker will " often " allow the attending Tech to correct the lower level fault before the most expensive part of the machine is destroyed. I have seen this countless times and the commercial trend in the state I came from was definitely away from using fused disconnects on that type of gear.

I can see fans or centrifugal pumps continuing to run under single phase conditions, but higher torque, low inertia loads which should include a refrigeration compressor, should not be able to continue to provide enough torque under phase loss conditions and will result in high enough current on remaining phases to take out a second fuse fairly quickly. Take out two phases and the motor no longer has any current flowing through it, unless there maybe is a ground fault involved and the remaining phase is supplying current through this fault path. Phase loss monitors are not a bad idea even if you have circuit breakers for overcurrent devices, as a lost termination somewhere or phase loss from utility side of things will still do the same thing.

 

[petersonra]

We have standardized on class CC fuses for up to 30A and class J fuses for > 30A for power circuits using fuses. The least expensive option and takes up the least amount of space, and 200kAIC. For 30A and 60A sizes they also come in DIN rail mount so is easy to work with. Also "touch safe".

My personal opinion is that one should not rely on a CB or fuse to trip on a single phase situation, as it can be unreliable. If this is an issue that you need to address, get some kind of device that detects single phasing to trip it. In the real world, single phasing is pretty rare, so I just do not worry all that much about it unless someone wants to pay extra.

 

[kwired]

In the real world, single phasing is pretty rare, so I just do not worry all that much about it unless someone wants to pay extra.

In the rural electric distribution systems, it is not all that rare, but I guess that depends on how frequently it has to happen before you call it rare vs common. Miles of lines out in the elements are bound to have a problem here and there from time to time.

 

[iwire]

In the real world, single phasing is pretty rare,

I agree with your post other than what I quoted.

We respond to a large number of single phasing events each year. Most common is the utility dropping a phase during a storm. Often this will last for many hours.

At this point most of our customers have phase loss protection on larger 3 phase equipment but we almost always end up replacing at least a few contactor coils that burn out due to under voltage resulting from these lost phase events.

 

[petersonra]

I agree with your post other than what I quoted.

We respond to a large number of single phasing events each year. Most common is the utility dropping a phase during a storm. Often this will last for many hours.

At this point most of our customers have phase loss protection on larger 3 phase equipment but we almost always end up replacing at least a few contactor coils that burn out due to under voltage resulting from these lost phase events.

how does single phasing result in under voltage that burns out contactor coils???

we get single phased here in our plant about once a year during storms. never had it cause any damage. maybe our systems are better protected than others, but AFAIK we have done nothing unusual to protect ourselves from single phasing.

 

[iwire]

how does single phasing result in under voltage that burns out contactor coils???

Back feeds through other equipment.

 

[GoldDigger]

Back feeds through other equipment.

Such as motors that act as very poor rotary phase converters, for example.

 

[petersonra]

Back feeds through other equipment.

How does it burn out the COIL?

 

[GoldDigger]

How does it burn out the COIL?

In a mechanically latching relay, the contact assembly may not move to the position where the coil circuit is interrupted.
Also the current in a continuously energized coil will vary with the armature position. (The inductance changes because the magnetic circuit changes.)

 

[iwire]

How does it burn out the COIL?

120 volt coil supplied with less than 120 volt for hours on end.

Maybe it has more to do with the 480-208Y/120 transformers being single phased than back feed through other equipment I really don't know.

What I do know is it happens often at many locations.

 

[iwire]

In a mechanically latching relay, the contact assembly may not move to the position where the coil circuit is interrupted.
Also the current in a continuously energized coil will vary with the armature position. (The inductance changes because the magnetic circuit changes.)

These will generally be magnetically held lighting or motor contactors.

The mechincaly held will do fine unless they try to change state during the event.

If they are IEC style contactors we often have to replace the entire contactor because the overheated coil will melt and deform the contactor it is in.

We often find them by smell.

 

[just the cowboy]

Hello,

15amps is the min circuit amps for a/c unit. RLA is 49.50amps. 480V three phase.

My wired is sized for 15amps

If the RLA is 49.50 why 15 amp fuses and wire?????

 

[iwire]

If the RLA is 49.50 why 15 amp fuses and wire?????

I bet he means LRA as in locked rotor amps.