Fuse protected disconnect

[wireday]

I have a 480V 30 amp disconnect for a electric heater, on the inside cover it says this switch capable of 30 amps 600V max when protected by RK1 or J fuse only, break all lines, my actual load is 24 amps 480, I am feed from a 30 amp CB do I need to fuse this,

 

[don_resqcapt19]

I expect that there is more on the label and the fuse requirement is to get the short circuit rating shown.

 

[petersonra]

I have a 480V 30 amp disconnect for a electric heater, on the inside cover it says this switch capable of 30 amps 600V max when protected by RK1 or J fuse only, break all lines, my actual load is 24 amps 480, I am feed from a 30 amp CB do I need to fuse this,

Most likely the fuses are required to rise above the default SCCR rating of the switch as Don mentioned.

There is generally no requirement to fuse a disconnect switch but it might be convenient if you need fuses anyway, or if you need them to get to an appropriate SCCR.

What is the available short circuit current at the incoming terminals of the switch?

 

[wireday]

Not a lot on the switch or box it self, this is European, the instructions say RMS symmetrical Ampres class RK1 10000. Class J 65000 The unit is a Eaton model AHDS30. Would the CB interrupting rating cover it?

 

[petersonra]

Not a lot on the switch or box it self, this is European, the instructions say RMS symmetrical Ampres class RK1 10000. Class J 65000 The unit is a Eaton model AHDS30

This probably means with Class J fuses it has a SCCR of 65 kA, or 10 kA with Class RK1 fuses.

Whether you need that high a rating or not depends on what the available short circuit current is.

 

[wireday]

The panel is rated for 22k

 

[petersonra]

The panel is rated for 22k

That is just the rating of the panel. presumably the available short circuit current is less than that out at the switch. You could calculate it based on 22 kA at the panel.

 

[wireday]

Would that put me with J type fusses,between the beaker and service switch,

 

[petersonra]

Would that put me with J type fusses,between the beaker and service switch,

It would be best if you found out what the available short circuit current is at the panel feeding this switch and did the calculations to find out what it is at the switch.

By the way, the moderators usually ask posters who have not done so to update their profile to include their occupation.

 

[wireday]

It would be best if you found out what the available short circuit current is at the panel feeding this switch and did the calculations to find out what it is at the switch.

By the way, the moderators usually ask posters who have not done so to update their profile to include their occupation.

I am a master electrician I did have it on the profile when I signed up, must have been deleted, now I don’t see where it would go on the profile, I am on my cell though,

 

[petersonra]

I am a master electrician I did have it on the profile when I signed up, must have been deleted, now I don’t see where it would go on the profile, I am on my cell though,

I think it got deleted for everyone when the software was upgraded.

 

[ASG]

How do fuses provide any extra short circuit protection? Let's say you have a 5k unit. The kAIC at the switch is 13kAIC. The breaker didn't give it any extra protection so how does this fused switch?

 

[jim dungar]

How do fuses provide any extra shor circuit protection? Let's say you have a 5k unit. The kAIC at the switch is 13kAIC. The breaker didn't give it any extra protection so how does this fused switch?

If you have two devices in series, such as a fuse and switch or a breaker and switch, it is possible to have them tested as if they were a single item. The result is a series-rated Short Circuit Current Rating or SCCR which could be higher than the lowest single device.

AIC applies only to devices that interrupt fault current. Your example would be a switch with a 5kA SCCR applied on a circuit with 13kA short circuit amps (SCA).

 

[jap]

I have a 480V 30 amp disconnect for a electric heater, on the inside cover it says this switch capable of 30 amps 600V max when protected by RK1 or J fuse only, break all lines, my actual load is 24 amps 480, I am feed from a 30 amp CB do I need to fuse this,

If the switch came mounted on the unit, and, the unit required a 30 amp feed,,,, I'd stop reading about right here,,,, (480V 30 amp disconnect)


JAP>

 

[wireday]

If the switch came mounted on the unit, and, the unit required a 30 amp feed,,,, I'd stop reading about right here,,,, (480V 30 amp disconnect)


JAP>

 

[wireday]

Switch did not come with unit,

 

[jap]

Switch did not come with unit,

If the switch is not capable of being fused, it seems it would be more economical to find an appropriate disconnect for the unit rather than having to back a breaker with a fuse just to satisfy the requirements for the switch you have.

JAP>

 

[wireday]

Switch did not come with unit

 

[ASG]

If you have two devices in series, such as a fuse and switch or a breaker and switch, it is possible to have them tested as if they were a single item. The result is a series-rated Short Circuit Current Rating or SCCR which could be higher than the lowest single device.

AIC applies only to devices that interrupt fault current. Your example would be a switch with a 5kA SCCR applied on a circuit with 13kA short circuit amps (SCA).

I understand they can have a series rating but there is/was no mention when I see these fuses talking about them getting that protection due to series rating. I doubt they tested the fuses with every type of circuit breaker it could be in series with.

 

[jim dungar]

I understand they can have a series rating but there is/was no mention when I see these fuses talking about them getting that protection due to series rating. I doubt they tested the fuses with every type of circuit breaker it could be in series with.

In post #4, you said they had one rating for RK1 fuses and a higher rating for use with a J fuse. This means they have series ratings.