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Various Brands of Fuses Tested

robby

Photon Vampire
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May 1, 2021
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Even though his methodology is not really correct, it is an eye opener on some of the brands.
A big thanks to him for doing the tests. And yes we should all be using T Fuses / HRC fuses as the ones in his video can Arc over and cause a fire if a dead short happens.



Enjoy
 
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Clearly this guy, as he says, is not an expert. Quite the opposite, I would say!

First of all, any fuse should be able to carry its rated current essentially indefinitely at normal ambient temperatures. The fact that his first 100A fuse test blew the fuse in quite a short time at 100A is a big red flag. Likewise the breaker tripping in similar fashion.

One thing wrong with his methodology is he doesn't understand the implications of employing an inverter. If a "true" sine wave inverter, it will likely be drawing a current from the battery that looks like a full-wave rectified 50 or 60Hz sine wave. That is, the average and RMS (heating effect) values of the current will be different. I'm guessing his meter is average-responding as I did not see "true RMS" labelled on it. Roughly speaking, you could expect the RMS current to be 1.11 times the average value for this case - not a big correction, but you absolutely need to use a RMS reading meter for this (and not just any RMS meter, but one that correctly computes the true RMS value of DC plus AC current). And especially if you don't know the inverter characteristics ("true" sine wave, or modified sine wave, for instance). Here I'm assuming the inverter is connected to a resistive load (as I would expect the heaters to be - although not if they employ phase-controlled heating adjustment). But his power supply charging the battery will likely draw a current with a very peaky waveform, making matters much worse.

Another big red flag is that he expects a fuse to blow quickly even moderately above rated current. The Victron fuse looks like a MEGA fuse. (In fact, I think I can see "MEGA" written on the package label.) Assuming it's from the 32V series, looking up Littlefuse's time-current curves at


you will see that a 100A fuse will trip at about

1.2 seconds for 300A
9 seconds for 200A
about 100 seconds for 150A (hard to read the graph at this point).

So I would say his test (given the major uncertainty relating to not measuring RMS current) roughly confirms that.

Why the other fuses and the breaker trip much sooner, I don't know.


In fact, you actually want a fuse or circuit breaker to pass much greater than rated current for a short time.

First of all, the things you are protecting can generally handle much more current for a short period than continuously - that includes things like wiring, transformers, motors, etc. Typically the most "sensitive" items are semiconductor devices, which may require special fast acting fuses for proper protection.

But not only can most things handle more current for a short time, they may actually need to draw much more current during startup, etc. This applies especially to motors, but also to incandescent light bulbs, transformers (inrush current), rectifiers with large filter capacitors, etc.
 
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alan; That is the first I heard my psw inverter pulls a sinewave dc from the battery. Could you provide a link where I can understand this? Thanks.
Cheap fuses are cheap protection-level. I mean junk. Always use a quality brand fuse and a trusted distributor.
 
Not sine wave, but full-wave rectified sine wave (like you took the absolute value of a sine wave centered around zero).

See e.g., https://www.victronenergy.com/live/_media/ve.bus:4._ripple_in_a_ac_battery_system.pdf

For a bit more detail, search for "ripple" in


The current waveform on the DC side depends on the nature of the load on the AC side (resistive, non-linear like some rectifier power supplies or AC phase controllers, etc.) and the design of the inverter, but fundamentally, most single-phase inverters have a large AC component to the current they draw from the battery.
 
What are you saying? This inverter does not have large capacitors on the DC inputs?
 
The input capacitors used in typical inverters are way too small to support the 100 or 120Hz ripple current the inverter generates. They are there to support e.g., the high frequency currents internally generated by high frequency switching within a PWM (pulse-width modulated) sine wave inverter.

A simple calculation will show that. For the case in the video, assume the average DC current was 100A with an inverter output frequency of 60Hz. The peak value of that rectified sine waveform is then pi/2 * 100 = 157A. The fundamental component (120Hz sine wave) of that waveform is 4/(3pi) * 157 = 66A pk. A 10mF (10,000 uF) capacitor has a reactance at 120Hz of 1/(omega * C) = 1/(2pi*120*0.01) = 0.13 ohm. If you passed that 66A pk 120Hz fundamental component of the ripple current through it, the voltage across the capacitor would be 2*66*0.13 = 17V pk-pk! To get the ripple voltage down to less than 1V p-p, you would a capacitor of order 170mF (170,000 uF).

In practice, the source impedance of the battery is much less than the parallel impedance of the inverter input capacitors at 120Hz, and it is the battery (plus battery to inverter wiring) impedance that determines the 120Hz ripple voltage. Most of the ripple current (at 120Hz and lower harmonics) goes to the battery with a small part going through the input capacitors.

OTOH, at the inverter internal switching frequency (around 20kHz for the Victron inverters in the examples I quoted above), the reactance of a 10mF capacitor would be 0.8 mOhm (0.0008 Ohm). Now that capacitor is dominating the source impedance seen by the inverter switching circuitry, and the high frequency switching currents go mostly through the input capacitors. (In practice, the input capacitor impedance won't be quite that low because of losses within the capacitor, expressed as the effective series resistance [ESR] of the capacitor in manufacturer's specifications.)

YMMV, depending on inverter design.
 
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