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# Help with HV fuse selection

#### maysin

##### New Member
Hi everyone, I am planning to use a 700V DC/63A fuse for my 50A DC current application. The fuse manufacturer insists that it will not be suitable for long term use and asks to use 1000V/80A fuse instead.
Surprisingly, I have been using this fuse for 48A AC application and it's working great. However the manufacturer insists that continuous DC current use is more riskier. I am not able to understand this logic. Wasn't RMS AC calculated with assumption of same power that the DC current would have produced. Am I missing something here?

It's easier to break AC current because the voltage crosses zero volts twice per cycle. If there is an arc across, it will be broken when the voltage crosses zero.

With DC, you don't get the zero crossings to extinguish an arc. If an arc starts across the fuse, there's nothing to extinguish it. DC-rated (high voltage) fuses have more separation between contacts and/or are filled with sand around the fuse element to prevent an arc from starting

True, but just want to point out that in this case the fuse is already rated for DC (700V/63A).

I guess, my question is more towards understanding the differences in fuse long term operation with AC vs DC current?

Is DC current continuous usage worse than AC? Not able to understand why. Maybe internal corrosion is more severe, don't know

True, but just want to point out that in this case the fuse is already rated for DC (700V/63A).

I guess, my question is more towards understanding the differences in fuse long term operation with AC vs DC current?

Is DC current continuous usage worse than AC? Not able to understand why. Maybe internal corrosion is more severe, don't know

As already explained, fuse ratings are usually MUCH lower for DC than AC, for the stated reason.

Why not comply with the manufacturers recommendation?.

It's not the current; DC current produces the same heating as the same AC RMS current.

As others have said, it's the arc effects as the circuit is broken.

Look at just about any switch, relay contact or contactor ratings etc. and the permitted DC working voltage is always significantly lower than the AC voltage.

Something I've had direct experience with recently, specifying parts for an unusual job, requiring power contactors; the Schneider LC1D series we normally use for AC controls have vastly lower ratings at DC than AC:

eg. One rated to 40A for inductive motor loads at up to 690V AC can switch:

ONE AMP, at 250V DC per contact! At somewhere around 100V DC, the single contact ratings drop off a cliff. Despite the 690V AC rating mean the peaks on that can be near 1000V.

You have to use three contacts in series to get back to 40A rating at just 250V DC! They cannot be used above 300V DC at all.

Dedicated DC switching contactors for higher voltages (rather than 12/24V etc), which used to be common but are rare on anything other than vintage machines, have much bigger contact gaps and electromagnetic "Arc chutes" - a coil in series with the contacts and pole pieces either side of the contact gap, that "fire" the arc plasma away the contacts as they open!

I see that you are not convinced and yet you haven't shared what load impedance could cause excess current. Wires are inductive, motors are very inductive and inverters chargers can be inductive. This affects the arc voltage V=LdI/dt where a dry fuse break can very fast then the fuse arcs after it breaks.

There are many types of fuses and this is a question, are you trying to protect the fuse or your system? What energy failures can a fuse fix? not fix?

Have you heard of semiconductor fuses? Until you share more info, I suggest you compare all the specs and costs of failure. Fuse specs include; holding current, clearing energy defined by I^2t, and Power Losses.

I recall a major US grid field engineer (Duke) who was told to fix a Huawei Inverter with the same to use 100A fuses when 70A fuses started blowing for 50A HV inverters after a bunch of competitor wind farms were operational. I suspected harmonic ingress currents and unstable disturbances. Huawei Inverters are very high-quality designs.

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