Fuses vs Breakers question

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RandyL
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Fuses vs Breakers question

Post by RandyL » Fri Oct 27, 2017 5:23 pm

For an allin1 cnc conversion what is the prefered safety device for circuit protection? Fuses or Breakers? Same amp rating. Is there a difference?
I would think breakers are better as they are resetable.. :roll:

cncsnw
Posts: 328
Joined: Wed Mar 24, 2010 5:48 pm

Re: Fuses vs Breakers question

Post by cncsnw » Fri Oct 27, 2017 7:14 pm

It mostly comes down to personal preference.

In years past, when UL489 branch-rated protection was required for smaller loads (say 30A and below), class CC fuses were less expensive and required less panel space.

Now that compact UL489-rated circuit breakers are available (e.g. the Eaton FAZ-...-NA series), I prefer to use circuit breakers for everything.

As you point out, they are resettable, which saves time and money when a problem arises. They also make it more convenient to turn sections of the system on and off during testing (though that is easy enough with flip-open class CC fuse holders as well).

RandyL
Posts: 9
Joined: Fri Sep 15, 2017 1:53 pm
Acorn CNC Controller: No
Allin1DC CNC Controller: No
Oak CNC controller: No
MPU11 & GPIO4D -w/ 3rd Party Drives: No
DC3IOB: No
CNC11: No
CPU10 or CPU7: No
System Serial Number: none

Re: Fuses vs Breakers question

Post by RandyL » Sat Oct 28, 2017 12:23 pm

I bought some of these ROGY JVM16-63, Class C rated 20 amp din rail mount.
I think they are same style you referred to (e.g. the Eaton FAZ-...-NA series)

https://www.ebay.com/itm/JVM1C20-Rogy-D ... 2749.l2649

I am using allin1, maybe 20 amp class C is to high amp rating to power 230 vac step down transformer? Maybe should use 15amp..

also, is class C appropriate?

cncsnw
Posts: 328
Joined: Wed Mar 24, 2010 5:48 pm

Re: Fuses vs Breakers question

Post by cncsnw » Sat Oct 28, 2017 9:46 pm

By my understanding of the code -- and I am not a licensed electrician or a code expert -- you are supposed to use branch-circuit-rated protection for motor power circuits. That would mean either branch-rated fuses (such as class CC, class J, class T or class RK5); or a UL489 circuit breaker.

The circuit breaker you indicated appears to be a UL1077 supplementary breaker, not a UL489 branch-rated breaker.

As for whether 20A is a suitable rating for a given transformer: see NEC 450.3, and in particular table 450.3(B). To correctly size the primary-side protection, you need to know the VA (or KVA) rating of your transformer.

For example, Centroid's "high power" transformer, used with servo supply PN10010, is rated 2.3KVA, or 2300 VA. If you supply that transformer with 230VAC on the primary side, then the rated primary current would be 2300 / 230 = 10.0A. Referring to NEC 450.3(B): if you only protect the primary side, you are limited to 125%, or 12.5A (which you are allowed to round up to the next commonly-available size). If you protect the secondary side as well (i.e. put fuses or circuit breakers on each secondary output), then your primary-side protection can go up to 250%, or 25A. In that latter case, you would need to protect the secondary windings at 125%.

Note that it is normal practice -- and is probably a requirement somewhere -- that you protect both primary wires leading to the transformer. So you will want a 2-pole circuit breaker for that purpose.

I typically use a 20A branch-rated fuse or breaker on the rectified DC output going from the CAPBRDHI to the All-in-one DC unit, but I have not found a published input current rating for the All-in-one. Perhaps someone from Centroid could supply that value.

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