Fuses - how many and where?

[Richards]

Yesterday, I rewired a bunch of stuff on my test bench. Typically, I use 14 gauge wire between the wall socket, through the disconnect switch an then to the line size of the circuit breaker. The circuit breaker that follows the disconnect is 10A because I use 16 gauge wiring between the circuit breaker and the various transformers. There is very little load at the test bench. The wires stay at room temperature. Depending on the chart used, 16 gauge wire can handle 22A or as little as 3.7A. That same chart shows that 14 gauge wire can handle 32A or as little as 5.9A. 14 gauge is typically used in household wiring for 15A circuits.

Anyway, I started thinking about fusing. Depending on the load, I usually put a fuse or circuit breaker just before a power supply and another fuse after the power supply, depending on the type of load. For example, I use a 2.5A DIN rail mounted 24VDC power supply. On the output side of that power supply, I use a 2.5 (maximum) fuse between the power supply and a 16-position terminal block. That fuse protects the power supply if anything connected to the DIN terminal blocks short out; but, should it be the only fuse that I use? How about the cables that connect proximity sensors to the test bench? The proximity sensors that I use require +24VDC and GND. They return a signal on a third wire. I probably should fuse each proximity sensor because a broken/shorted cable could cause the +24VDC and the GND wires to touch. Without a supplementary fuse, if a proximity cable shorted, the fuse protecting the power supply would blow and all +24VDC devices would have no power. That's not allowed for medical devices. It doesn't seem to be a good idea for industrial control. What if I replaced the proximity sensor with a standard limit switch - but miswired the switch so that +24VDC shorted to GND when the switch activated? Wiring mistakes happen.

Another example is whether to fuse the output of a Teknic IPC-5 power supply used to provide power to ClearPath servos. The ClearPath User Manual on page 34 tells us that the IPC-5 power supply has no internal fuse on the DC output side. We're only told that if a fuse is required to meet compliance standards, that it should be installed on positive leg. That's a given. What size fuse? How about the power regenerated by a servo when it decelerates? Should I even worry about it?

I've scanned through some UL and NEC papers, but I haven't found what I'm looking for yet. Lot's of questions but few concrete answers. Usually it wouldn't matter, but if improper fusing caused a fire or personal injury, what would the insurance company pay?

NOTE: After posting this, I checked some documents supplied to me by a company where I was a consultant. Their electrical engineers fused every conductor that carried power that exited the control box. For instance, all sensors were fused. All remote switches were fused. Everything leaving the control box had a supplementary fuse between the out-of-box device and its power supply inside the control panel.

[Centroid_Jacob]

Hey Mike,
Great questions, your last few statements are definitely correct. Check out this schematic to see how we do it in house:
https://www.centroidcnc.com/dealersuppo ... 031.r2.pdf

Oh by the way we really like using circuit breakers, be sure to use the D curve type circuit breakers for motor controllers such as Axis Drives and VFDs.

[cncsnw]

If you are remotely interested in going the extra distance with circuit protection, you will want to take "how Centroid does it in house" with a big grain of salt.

For example, if you follow the schematic Jacob posted, you will see that the lube pump, supplied with nominal 220VAC (typically 240VAC in North America), has for its overcurrent protection only the 15A time-delay fuses F3 and F4. This in spite of going through a relay that is probably rated for only 5A, and in typical factory practice being wired with 18ga wire.

Similarly, in a typical factory installation, transformer T1, supplying 24VAC, is a 100VA transformer. NEC 450.3(B) says that the primary-side overcurrent protection in this case should not exceed 300% (or 250%, depending on how you read it) of the rated current. Rated current would be about 0.45A on 220V. 300% would be 1.36A. You are allowed to round up to commonly-available fuse or breaker sizes, so a 1.5A circuit breaker or fuses would be appropriate. However, that transformer primary is typically also protected only by 15A fuses.

I usually do not go so far as to install a separate supplementary fuse or circuit breaker on every ungrounded wire that leaves the cabinet, but I do include a lot more protection in the power-distribution areas than you will find on an in-house schematic.

For PLC input circuits (e.g. limit switches and proximity sensors) I usually count on the switching power supply to protect itself in case of short circuits. Here is it nice to have a 24VDC supply for inputs that is independent of the logic supply that keeps the Centroid control board alive. That way your control and PLC continue to function, even if a short circuit is causing the inputs supply to shut down. For improved troubleshooting ability, you can wire one PLC input directly from the supply bus, and add PLC logic to trigger a Fault (of higher priority than any other fault) to report that the inputs supply is offline or shut down.

[cnckeith]

thanks Marc! yep our system schematic is general guidance, a starting place if you will, as it is up to the installer to make these types of decisions and follow any codes/regulations that apply to their particular CNC project.

[Richards]

Thanks everyone (and also those who texted/emailed me to offer help). What I think I'll do is to follow the general guidelines in Centroidcnc's https://www.centroidcnc.com/dealersuppo ... 031.r2.pdf schematic with modifications to suit my controller. Wiring will be sized to handle the amps flowing through the circuit. Each fuse/circuit breaker will be sized for the load that follows the fuse/circuit breaker. All power conductors exiting the controller will be fused. Adding fuses/circuit breakers to fully protect the controller might add $100 - $200 to the total cost.

Years ago, before I got involved in electronics, I owned a professional photo lab that served most of the professional wedding and portrait photographers in the Salt Lake City area. One of my friends and competitors also owned a photo lab that served commercial photographers. His lab technician used fish tank heaters to keep the chemical baths at proper temperature. The fish tank heaters were designed to raise the water temperature just a few degrees in a home/office environment. One cold winter night, as the room temperature dropped and dropped and dropped, the fish tank heaters worked harder and harder to keep the chemicals at proper temperature. The insulation on the "hot" conductor melted, the bare wire started a wooden wall on fire. He lost his entire business. Insurance paid because the fish tank heaters were designed to be plugged into an ordinary 120VAC outlet. A fuse rated to blow if the current exceeded the wire current capacity would have protected his business.

[Blaze]

Hello all. I was trying not to ask questions concerning circuit protection because I thought it’s elementary. Obviously not.., because I am struggling with it. I have had great help from Mr. Shawn thus far and I am trying to not keep fetching water out from the same well in hopes of eliminate the possibility of drying up the well. I am looking for suggestion and understanding that way I can solve this issue should it present itself moving forward.
I have a fully nested Router machine imported from China. I am in the process of converting it to Centroid Acorn Six Controller. I have a 240 VAC 32A breaker/ main power going to L1/T1 (+) and L2/T2 (-) off my 18vac coil Chnt NXC-18 Contactor to an EMI filter. From this filter to my DZB 7.5 HP VFD on a 14AWG. Is it necessary to add an other filter just before my power supply.

240VAC from contactor input L3/T3output to Stepdown Transformer @ 70vac for my 6 Leadshine H2-758 Drivers. (Not fused) on a 14AWG
240vac fromL3 (+) & 13NO (-) to my Terminal block to power my NVVV MDR-60-24 AC to DC DIN-Rail Power Supply 24V 2.5 Amp 60W and a MEAN WELL MDR-20-5 AC to DC DIN-Rail Power Supply 5V 3 Amp 15W. (none of these are fused protected) on a 14AWG.

I intend on using the 24VDC to power acorn and the relays. The 5vdc power supply to power acorn six 5vdc logic. I do not know how to select the appropriate rated fuse or circuit breaker for these circuit. I am looking to incorporate a breaker inform of the power supply and the fuse after.

Omron Prox. Sensors. I have 8 total @300mA Max 6-36VDC I will be using additional 24VAC @2.5a same as the one above on a 18AWG. My approach is 300mA*8 = 2.4watt 24vdc/2.4watt= 10A. Am I correct?

I looked at post addressing this topic. I saw for the 1st time Acorn Suggestion found on G6 of the schematic. I will review it some more. To gain a better understanding.

S15031.r2.pdf

(400.79 KiB) Downloaded 11 times

[Richards]

What I'm doing is installing a fuse or circuit breaker on the input side of each power supply and a fuse or circuit breaker on the output side of the power supply. In addition, I use a fuse on each output circuit that exits the control box.

For example, the MDR-20-5 power supply is rated at 0.55A at 120VAC input or 0.35A at 240VAC input. I would use a 0.5A slo-blo fuse or a 0.5A D-Curve circuit breaker for my 120VAC system. That power supply is rated at 3A, so on the output side, I would use a 3A slo-blo fuse or a 3A D-Curve circuit breaker. In my system, no 5VDC circuit exits the control box, so I wouldn't use additional fuses on the 5VDC circuit.

For the MDR-60-24 power supply, I would use a 1.8A slo-blo fuse or 1.8A D-Curve circuit breaker on the 120VAC input side and a 2.5A slo-blo fuse or a 2.5A D-Curve circuit breaker on the output. In addition, because the proximity sensors are outside the control box, I would use a 300mA fast-blo fuse in series with each sensor. AutomationDirect sells the DN-F6MN fuse holder. Other vendors have the same fuse holder in stock at a discounted price.

I use either DIN rail fuse holders similar to the Edison EHM1DU for midget fuse or D-Curve circuit breakers on each side of the power supply. Those fuse holders and midget fuse cost about the same as a UL 489 type circuit breaker. It is my understanding that the lower cost UL 1077 supplementary potectors should only be used to protect a single device.

My approach might be ultra-conservative, but everything is protected.