question: fuse rating

[Ingber]

Hi,

I have just ordered a Crack 1.1 and now I am reading the manual.
Having no understanding of the circuitry, I so far focused on some basic safety aspects like fuse, grounding etc.

I have read that a fuse's rating can be determined from dividing the maximum power consumption by the mains voltage and then adding some 10-20%.
In the case of any inrush current the rating should be higher. This would suggest that for 230V mains one needs a 0,5A fuse instead of the 1A fuse that comes with the kit. However, Paul Birkeland somewhere on this forum replied that 1A was low enough, also for 230V mains.

1) My question is therefore what is the maximum power consumption of the Crack? I read in the forum that when idle it is roughly 30-35 Watt, but how much is it in operation?

2) A second question would be if there is any inrush surge current when switching on the Crack and how much?

3) How the fuse protects in case of a fault in the primary winding of the transformer I can understand. But, will a fault (live wire accidentally connecting to neutral or to the grounded chassis) in the secondary winding of the transformer, i.e. the signal amplification circuit or the heaters, also create a high enough current in the primary winding to blow the fuse?

To enhance safety, I plan to screw the top plate (chassis) to the wooden housing. In addition, I plan to cover the bottom of the housing with a perforated metal plate that will be connected by a wire to the chassis ground.

Regards,

[Paul Birkeland]

The idle operating power and the power consumed while you have music playing are the same, as the Crack is a class A device. 

Yes, there is inrush current.  It can be a little bit of a pain to measure the instantaneous peak current, and remember that a 1A fuse won't blow from 1A of peak current, so these waters get muddy in a hurry. 

If a live wire connects to ground, you will trip a GFCI circuit, or blow the fuse when enough current is drawn.  For a live wire shorting to neutral, an AC mains wire doing this will blow the fuse instantly, and a high voltage DC connection from the power supply contacting neutral will also open up the fuse relatively quickly.  The low voltage winding is not all that well protected, as a lot of power can be drawn through that winding before the fuse pops, and in practice the power transformer will die before this happens.  This is why we have the glow test early on in the manual so you can get this part right when the rest of the amp isn't present. 

[Ingber]

Thanks Paul.

1) OK, clear.

2) I guess that the choice of fuse amperage for the Crack is more based on experience than on formulas. Is that also what determines if it has to be slow-blow or fast-blow?
Is such a fuse mandatory for an amp like this (if it was mass produced) in the US, or is it just good practice?

3) What I understand from your answer is that the DC high voltage power supply is protected by the fuse, but that the low voltage heaters circuit is not. I assume that this low voltage is not harmfull for humans.
You mentioned a GFCI. I have checked and it turned out that the old appartment that I am renting does not have one. I might get one installed. If I am correct, a ground fault in the secondary winding cannot be deteced by a GFCI because this part is "transformer isolated", right? 

Regards,

[Paul Birkeland]

The choice of fuse for the Crack is more based around experience and the PT-10 itself.  For 120V mains use, a 3A fuse even likely blow from a fault in the circuit, but a 0.5A fuse might eventually blow from turn-on surge.  This leaves a large range of fuse values that are appropriate for the project, and a 1A fuse happens to overlap for 120V and 240V operation.

There is a threshold for current flow to earth ground that will trip a GFCI.   If you power up a power transformer and grab each end of the primary with each of your hands, then there isn't any protection for that.  Having a GFCI outlet isn't a requirement for building a kit.

The fuse is not present to protect you from electrical shock, but rather it's there to protect the power transformer from faults that exist within the circuit.  If you make a mistake like installing one of the 220uF/250V caps backwards, the fuse will blow pretty quickly under those conditions.  If you don't have a fuse in the amp, then one of the 270 Ohm 5W resistors will smoke and burn until it opens up, or the cap that's installed backwards will heat up and swell until it explodes.

There are lots of written standards and regulations regarding fuses that apply depending on where you are and what you're doing.  It would be a very, very bad idea to not include the fuse with your build.