Author Topic: Fusing the power supply input  (Read 910 times)

jeremy0203

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Re: Fusing the power supply input
« Reply #15 on: October 08, 2018, 04:53:54 PM »
The AC side of your power supply surely has a fuse in your house electrical box.
Just a word of warning.  Draw 25 amps through a 20 amp breaker in your house breaker panel sometime and see how long it takes to trip.  It will scare you.  For most breakers it will be measured in minutes - maybe quite a few.


If you can pull 25A thru a 20A breaker for more than about 1 second, you have a defective breaker and the magnetic trip for the breaker has failed.  Heck, even pulling a steady 19A from a 20A breaker should cause it to trip in a few minutes.  That is due to the thermal trip protection of the breaker and is the reason for the 80% rule.  The breaker being the fuse or protection for your power supplies input side is also completely false.  The breaker is in place to protect the outlet and wiring between the breaker and the outlet, not the device(s) plugged into the outlet.  Putting a fuse on the input side of a PS doesn't really do all that much and in most cases is simply not needed.  Putting fuses on the output side of the supply is a good idea and will protect the supply from excess current draw that can overheat or damage the supply.


You can pull 1.25 times the breaker rating for almost a minute with most standard residential breakers. Pulling the rated current it will trip within the 5 min range

Offline k6ccc

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Re: Fusing the power supply input
« Reply #16 on: October 08, 2018, 05:25:04 PM »
If you can pull 25A thru a 20A breaker for more than about 1 second, you have a defective breaker and the magnetic trip for the breaker has failed.  Heck, even pulling a steady 19A from a 20A breaker should cause it to trip in a few minutes.  That is due to the thermal trip protection of the breaker and is the reason for the 80% rule.
Oh, you are SO wrong.  Take a look at the time current curve for a circuit breaker some time.  Note, I am NOT trying to put you down, but rather to educate you.  I am using the Time Current Characteristics chart for a Siemens residential circuit breaker from the Siemens website as a reference.
According to the chart, to trip the breaker in one second requires a current between 5.5 and 10 times the rated current.  In other words, for a 20 amp breaker, to get it to trip within 1 second would require between 110 and 200 amps.  At 25 amps for that 20 amp breaker would take a minimum of 150 seconds and a maximum of about a half hour.  BTW, the chart assumes a temperature of 40 degrees C.

Note, with all that said, on your typical residential circuit breaker, you CAN cause it to trip at far less current if it's hot enough.  For example, at my house, the main breaker panel is on a southwest facing wall with absolutely zero shade.  It was also an old Zinsco panel which have a reputation for getting hot (and starting fires) because of crappy connections between the buss and the breakers.  I kept having the breaker trip on my central air conditioner compressor.  While investigating it on a blazing hot day, I had a clamp on amp meter on the wiring to the AC and while measuring 27 amps, the 30 amp breaker tripped.  The panel was so hot from the 110 degree day and the direct sun, that you could not touch it without gloves.  BTW, I have subsequently replaced the old Zinsco panel (upgraded from 100A to 200A in the process too)...

The breaker being the fuse or protection for your power supplies input side is also completely false.  The breaker is in place to protect the outlet and wiring between the breaker and the outlet, not the device(s) plugged into the outlet.
That part you are correct.  Note that it was not me who made the statement.


Putting a fuse on the input side of a PS doesn't really do all that much and in most cases is simply not needed.
Actually what a fuse on the input of the power supply accomplishes is protecting the wiring between the AC plug and the power supply (and to a lesser extend the wiring between the breaker and the AC outlet).  Again using the example I cited above of using 18 AWG SPT-1 for a typical 350 watt power supply.  For most purposes, the SPT-1 is rated for 7 amps which is just fine for our 350 watt power supply that will max out at just over 3 amps.  If the power supply fails with a fairly hard short, the input fuse will trip LONG before the breaker in the electrical panel trips.  Lets say that short (with the resistance of the SPT-1) draws 100 amps.  It will take between 1.5 and 3.5 seconds for the 20 amp breaker to trip.  How hot do you think the 18 AWG SPT-1 will get in 3.5 seconds with 100 amps flowing through it?  The answer is REALLY HOT.  As in start fires hot... Fortunately (and I did not know this until pixelpuppy posted it a couple days ago) the power supplies apparently DO have an input fuse.

Putting fuses on the output side of the supply is a good idea and will protect the supply from excess current draw that can overheat or damage the supply.
Correct.  However if you direct wire the power supply to a pixel controller, there is no fusing.  A fuse on the power supply output would protect the power supply and wiring to the pixel controller in the event of a failure of the pixel controller (on the high current buss part of the controller).   There is normally fusing on the outputs from the pixel controller to the pixel strings, but you would be hard pressed to draw enough current from a shorted pixel string to blow a fuse on the output of the power supply.  You sure could damage the pixel controller however...  Hence the fuses on the pixel outputs on the pixel controller board.
« Last Edit: October 08, 2018, 05:41:22 PM by k6ccc »
Using LOR (mostly SuperStar) for all sequencing - using FPP only to drive P5 and P10 panels.
Jim

Offline sklankowski

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Re: Fusing the power supply input
« Reply #17 on: November 26, 2018, 07:21:12 PM »
I utilize these, spade terminals  on inlet power.

https://www.mouser.com/ProductDetail/571-W57XJ2A22B10-20M


 

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