DC Device Fusing
- Thread starter bobhudon
- Start date
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: DC Device Fusing
I believe the requirements do not differ from those for AC.
I believe the requirements do not differ from those for AC.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: DC Device Fusing
Breakers and fuses do not protect the loads. They protect the conductors that serve the loads. If the conductors can handle the current that your proposed "one fuse" will allow to pass, then you do not need separate fuses.
Breakers and fuses do not protect the loads. They protect the conductors that serve the loads. If the conductors can handle the current that your proposed "one fuse" will allow to pass, then you do not need separate fuses.
Re: DC Device Fusing
Charlie,
I understand that. Poor wording on my part. I have the conductors protected. My confusion is device protection. If the manufacture does not protect his device from internal shorts/overload then do I have to supply it?
If a device that normally draws only 80-ma is supplied by a branch circuit fused at 10-amps and the device shorts internally, the 10-amp branch fuse may blow, unlikely, or the device will fry until it opens the internal circuit feeding power to it.
Charlie,
I understand that. Poor wording on my part. I have the conductors protected. My confusion is device protection. If the manufacture does not protect his device from internal shorts/overload then do I have to supply it?
If a device that normally draws only 80-ma is supplied by a branch circuit fused at 10-amps and the device shorts internally, the 10-amp branch fuse may blow, unlikely, or the device will fry until it opens the internal circuit feeding power to it.
Re: DC Device Fusing
If the device is tripping the OCPD then the device is toasted (that's an engineering word). What is there to protect?
There are usually some listing requirements to set OCP such that the malfunctioning device does not start a fire. But that does not do anything to save the device. By the time the OCP trips the device is already gone.
carl
Charlie is right. The OCPD is not to protect the device, it is to protect the conductors to not start a fire.
If the device is tripping the OCPD then the device is toasted (that's an engineering word). What is there to protect?
There are usually some listing requirements to set OCP such that the malfunctioning device does not start a fire. But that does not do anything to save the device. By the time the OCP trips the device is already gone.
carl
Re: DC Device Fusing
I would like to dig a little deeper into this subject. If I size the OCPD to protect the wire feeding power to the device at say 7-amps but the internal wiring of the device can only handle say 1-amp then am I not still leaving the potential of a fire starting if the device malfunctions and continuously draws more than 1-amp?
I would like to dig a little deeper into this subject. If I size the OCPD to protect the wire feeding power to the device at say 7-amps but the internal wiring of the device can only handle say 1-amp then am I not still leaving the potential of a fire starting if the device malfunctions and continuously draws more than 1-amp?
rick hart
Senior Member
- Location
- Dallas Texas
Re: DC Device Fusing
You could calculate the maximum current under normal usage and place a fuse that will open before the component, I suppose but, a high resistance connection could also be a source of heat that could start a fire without proper containment.
All of this has to stop somewhere and in the age of disposable everything, what's the point. Protect the conductors so you can go to Best Buy for a new gizmo, re-connect to the existing circuit and go on with life!
.
The device at some point becomes a fuse- current will pass through some component that will open the path way. What you are trying to do is open a current path and extinguish the arc that results when that occurs. Like a power fuse that can open a high fault current, the device must provide a containment when the catastrophic failure happens.
You could calculate the maximum current under normal usage and place a fuse that will open before the component, I suppose but, a high resistance connection could also be a source of heat that could start a fire without proper containment.
All of this has to stop somewhere and in the age of disposable everything, what's the point. Protect the conductors so you can go to Best Buy for a new gizmo, re-connect to the existing circuit and go on with life!
.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: DC Device Fusing
Provide one fuse at the line end of the "feeder", and then divide using appropriate smaller fuses for each load at the load end.
If this is the case, I'd do both, just like a miniature power distribution setup.
Provide one fuse at the line end of the "feeder", and then divide using appropriate smaller fuses for each load at the load end.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: DC Device Fusing
For small DC loads with 18-20 gages wires, often the wire will become the fuse
For small DC loads with 18-20 gages wires, often the wire will become the fuse
Re: DC Device Fusing
I wanted to be sure that I was following code while at the same time keeping cost as low as possible. I would rather not fuse each load.
My current design uses a properly sized fuse to protect the feed wire from the 7-amp DC power supply. I have another 4-amp fuse on a branch circuit supplying power to a touch screen. The remaining branch circuits are not fused and each supplies power to a small ma load.
I wanted to be sure that I was following code while at the same time keeping cost as low as possible. I would rather not fuse each load.
My current design uses a properly sized fuse to protect the feed wire from the 7-amp DC power supply. I have another 4-amp fuse on a branch circuit supplying power to a touch screen. The remaining branch circuits are not fused and each supplies power to a small ma load.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Re: DC Device Fusing
There are a couple different things here.
1. The branch circuit wiring has to be protected.
2. The devices generally do not require individual protection as UL considers this when listing them. Most such devices are internally protected. A few will tell you to supply a fuse in the supply circuit NTE a certain value.
3. Many times it is convenient to have a fuse for each device so that the device can be unpowered for servicing w/o disturbing other devices.
4. It is also often a good idea to individually fuse devices leaving a control enclosure so that if a short occurs on that wire, only one device is affected. This does not always work because it is not as easy as you might think to coordinate the fuse sizing to make this happen. In a short circuit condition, it is not uncommon for multiple fuses in series to see a higher value fuse blow before a lower one. That is why when people do this they often use fast blow fuses on the devices and slow blow fuses on the main wiring.
[ January 06, 2006, 08:59 AM: Message edited by: petersonra ]
There are a couple different things here.
1. The branch circuit wiring has to be protected.
2. The devices generally do not require individual protection as UL considers this when listing them. Most such devices are internally protected. A few will tell you to supply a fuse in the supply circuit NTE a certain value.
3. Many times it is convenient to have a fuse for each device so that the device can be unpowered for servicing w/o disturbing other devices.
4. It is also often a good idea to individually fuse devices leaving a control enclosure so that if a short occurs on that wire, only one device is affected. This does not always work because it is not as easy as you might think to coordinate the fuse sizing to make this happen. In a short circuit condition, it is not uncommon for multiple fuses in series to see a higher value fuse blow before a lower one. That is why when people do this they often use fast blow fuses on the devices and slow blow fuses on the main wiring.
[ January 06, 2006, 08:59 AM: Message edited by: petersonra ]
Re: DC Device Fusing
petersonra,
The branch circuit is being protected by the feeder fuse. Branch circuit wires are sized to handle feeder current. If any branch current or sum of branch currents exceeds feeder fuse rating the fuse will blow. To satisfy code do I also need to add another fuse for each branch?
I realize it is best to fuse each branch for convenience purposes but is it actually required by code in the above scenario? Again, cost reduction is the goal.
petersonra,
The branch circuit is being protected by the feeder fuse. Branch circuit wires are sized to handle feeder current. If any branch current or sum of branch currents exceeds feeder fuse rating the fuse will blow. To satisfy code do I also need to add another fuse for each branch?
I realize it is best to fuse each branch for convenience purposes but is it actually required by code in the above scenario? Again, cost reduction is the goal.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: DC Device Fusing
Go with whatever is most cost effective. Are the devices really cheaper to replace than protecting them would cost?
Fusing electronics is usually done to protect the devices from the power supply, not the other way around, like we do.
If we're talking about low voltages, surmising from the 80 ma per device, I don't believe that code really applies.
Go with whatever is most cost effective. Are the devices really cheaper to replace than protecting them would cost?
Fusing electronics is usually done to protect the devices from the power supply, not the other way around, like we do.
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