# Voltage rating of series connected fuses

If a number of fuses are series connected, with the intention to achieve a higher voltage rating, what is the overall voltage rating?

For example if we place two identical fuses that are rated for 120V dc in series. I suspect the overall rating will not be 240V dc as one fuse will blow before the other.

Is the actual rating closer to 120V or to 240V? Is there some mathematical rule?

The example given is two fuses in series but what about n fuses connected in series?

• Think worst case scenario - only 1 of the n fuses blows, this leaves the voltage across it but its rating is still just the original voltage rating. Commented Feb 28, 2017 at 10:08
• DON'T. SERIES. CONNECT. FUSES. Commented Feb 28, 2017 at 10:16

The voltage issue with fuses is limiting the energy available to the arc as the fuse opens, it is not uncommon to see inappropriately chosen glass fuses explode when overloaded from a sufficiently butch source. Fuses have limits on safe breaking capacity, and for small glass examples these can be as low as a hundred amps or so, which is also due to arc energy limits.

If seen in these terms the voltage rating of a series chain is clearly that of the lowest voltage fuse.

An important thing to note is that fuse voltage ratings for DC service are often very much lower then the rated voltage for AC service as the DC arc will not self quench.

Where series connected fuses can be useful is when you have a very stiff supply, like say a telecomms battery bank, if you want to fuse something close to the batteries at say 0.1A, you have a problem, because the prospective short circuit current can far exceed the rating of the biggest 0.1A fuse you can find. Lets say you have a PSC of 50kA... What you do is place a much higher current fuse before the little one, coordinated such that the big fuse will catch nearly direct short circuits and limit the energy delivered to the arc in the much smaller fuse.

Fuses are NOT simple devices, read the datasheets carefully.

The voltage rating of a fuse is based on the voltage it can withstand when open (since the voltage across the fuse before it blows is insignificantly small). The rating of fuses in series will be that of the lowest.

• If a fuse is blown its breakdown voltage could be huge .Maybe an old school glass car fuse could take thousands of volts .If it was in the process of going open then it would only be safe up to its voltage rating . Commented Feb 28, 2017 at 4:18

As said above:

The voltage rating of a fuse is based on the voltage it can withstand when open

If the rating is exceeded, the fuse may arc then explode.

If you wire several low-voltage fuses in series, and if one blows and starts to arc, it will explode before the others have enough time to blow.

The only acceptable scenario to series connect fuses or circuit breakers is as in house wiring, for example: 50A general breaker feeds a row of 10A-16A-20A breakers, each protecting a separate circuit.

In this case, you do have 2 breakers/fuses in series, but all breakers/fuses are rated for the full voltage, and the one with the larger current is only there to protect the wires between it and the next, lower current breakers.

Fuse trips on current, when we series fuses - one with lowest current rating will blow leading to circuit break so despite your input voltage is low a short circuit in load will blow everything out.

For voltage protection look for voltage clamping zener diodes

• This does not answer the question regarding the voltage rating of series connected fuses. Commented Feb 28, 2017 at 10:17