Type R motor fuses

[rockman7892]

For all of the MV 4.16kV motors in our plant we use Simens draw out contactors for the means of motor starting. This motor starter consists of the draw-out contactor with vaccum bottle contacts, Type R motor fuses on the primary of the contactor, and a motor protection relay (Multilin 269) for motor overload protection.

I have been focusing lately on fuse selection for these type R fuses. Typically I follow the Siemens application guide which reccomends a type R fuse size based upon the FLA, LRC, and acceleration time of the motor. These values usually lead directly to a fuse size to use in this starting arrangement.

From everything that I have read these fuses are stricly for short circuit protection of the circuit. We do not use the instantaneous settings on the motor relay based upon the fact that the contacors are only rated for interrupting up to a certain fault current. So from a TCC plot standpoint we let the fuse take over at the bottom of the realy curve to interrupt any fault current.

As I look at the TCC I notice that the only thing that the fuses are protecting to the right of the curve are the cables to the motor. With that being said I have wondered what the negative effect would be for selecting/using a fuse that is a higher rating than what the application guide reccomends. If for theoretically some reason we were in a pinch and did not have the correct fuse size and only had a larger size avaliable what would be the risk of using this larger fuse size since it is only used for short circuit protection, and this fuse still fell to the left of the cable damage curves?

I know the negative effects of using a type R fuse that is too small b/c I have blown several of them in the past but am curious what is being risked by using ones that are too big since the relay provides the overload protection.

 

[dpc]

The only downside is that the larger fuse will be slower over a certain range of fault current. But you're right, the fuse is not there to protect the motor, it is to provide short circuit protection and prevent a fire. If there is problem in the motor itself, by the time the fuse clears, you will probably need a new motor anyway.

 

[lansford]

The R fuse is also selected based on the withstand rating of the contactor since the withstand rating of the contactor is not very high.
JIM

 

[LionelHutz]

Yes, you have to watch the withstand rating of the contactor too. However, most 400A+ rated contactors are suitable for use with R rated fuses up to 36R.

On top of that, you have to make sure there actually is enough fault current available to clear the fuse. A 36R fuse on a weak system may not be working in it's short circuit operating range. It's possible you will cause a long duration fault that could cause the contactor to fail or melt the fuse.

 

[rockman7892]

I have been trying to figure out the main difference between a "Type R" motor fuse and a "Type E" current limiting fuse.

I went ahead and plotted an 100A rated fuse for both types of fuses and do no see much of a difference on the TCC plot. I also looked at the specifications for each of these fuses and saw that they had the same interrupting ratings. The fuses that I plotted in the attached plot are a 3R fuse rated at 100A and an 100-E CS-3 fuse rated at 100A. The only difference that I notice is that the 100-E fuse extends higher and is further to the right.

I didn't think that R-type and E-type fuses could be used interchangebly but I do not see much of a difference on this plot. Can an R-type fuse be used as an OCPD for a transformer like I have seen in some distributin systems in the past?