A 30A breaker (on the dryer circuit) will trip in case of short circuit, and it is sufficient to protect wiring if the short circuit is high enough for tripping. The dangers remaining are :

1. A short circuit that is not high enough to trip the breaker, so can keep going for some time (even if only seconds). It takes 300A+ to trip a 30A breaker immediately by electromagnet, anything below that will last for as long as it takes the breaker to trip by heating bimetal strip - that is single seconds in high currents, up to minutes or hours in just a light overload

2. When messing with the powered circuit directly, and making some short circuit accidentally, if the available short circuit current is high it can make a small explosion (in high power industrial equipment it can be big one too). If you are too close to it you can get injured. The breaker (or a fuse) does not limit the peak current of the short circuit, it only determines the time the short circuit condition will last, so only partially determines the possible size of the arc



By using a lower rated (5A 10A 15A) breaker or fuse for the setup you lower the current that it takes to remove power immediately. Small fuses tend to react especially fast

But you have to check the breaker / fuse's interruption capacity. That is the highest short circuit curent it can handle safely, without the breaker being damaged from the arc, fuse exploding etc. The capacity is stated in the datasheet and can go from mere 10A..30A for a small glass fuse (which is not meant for anything besides electronics) to 1000's A for larger sand-filled ceramic fuse. For breakers the capacity is stated on the breaker and is normally in the 1000's A range

The breaker / fuse's capacity have to be sufficient to protect from the max short circuit current expected in the circuit, and that does NOT depend on upstream breakers



The actual short circuit current is set by the circuit impedance i.e. the transformer and the wiring all the way to the receptacle. The location of the short circuit in the system determine the size of arc

A related thing : The location in the system also determines the expected value of surge. Same as expected value of short circuit current, the value goes down the "deeper" you are into the home, on longer circuits with thinner conductors. This is the base of multimeter measurement safety classification - what happens if a surge happens while you measure something, with the multimeter and test leads in your hand. The multimeter have to be built to isolate the possible surge

Worth reading : http://media.fluke.com/documents/6002399-0000-eng-a-w.pdf

If we compare our experimetns to the measurement classes, we would like to make our experiments in the class II area. The dryer socket can be sometimes class III



Transformer impedance is extremely low and can be ignored, the most part come from the wiring and that can be calculated :

Ignore anything up to the panel, that is usually made of very thick wire anyway, and the transformer in the US is often right outside of the house so the service drop is short and its impedance is low

Lets say the way from the panel to the dryer socket is 10ft. As the short circuit takes the path in both directions, the actual length of the current path is double that - 20ft = 6 meters. The cable all the way is 10AWG = 5.25mm^2

Copper resistance is about 0.0168 ohm for 1m long section of 1mm^2 area

0.0168 * 6 / 5.26 ~ 0.019 ohm
240V / 0.019 ohm = 13000A

That is still high. Lets see what it takes to drop that to the order of 1000..1500A

240V / 1500A = 0.16 ohm

We want to add 0.14 ohm to the circuit. Lets say we use long 14AWG - 2.08mm^2 extension cable (through which both Phases go) from the socket to the test setup, and that will add impedance

2.08 * 0.14 / 0.0168 = 17.3 meters = 57 ft

as 57ft is the length of 2 conductors, the length of cable required is 28ft

That indeed is about the distance specified in that PDF file (table in bottom of 1st page, class II is 30 ft from class III)



One more thing, a GFCI will not hurt either. If you cannot find a US GFCI for 240V, use an European one (here its called RCD), with trip current of no more than 30mA. a 3 phase 30mA RCD can be converted into a 1 phase 15mA RCD by connecting its poles in series pairs (in the same direction)

I'm using my hot tub circuit which I'm pretty sure is 50a. I'll have to check the breakers though to be sure.

Well keep in mind u are dealing with very large current
For experment those small wires need to be properly fused. Well i got plenty of 240 volt lighting here
That i imported in. Well i have a 125 watt pop pack
240 volt 40 watt fizzy light pack LP40
Yes they can be safely use at home on single pole switch using 120-0 out of 120 -0-120 to operate a double pole relay. And planning on running 15 amp
120-0-120 circut to my room for expermenting
I got a 240 volt GFCI plug that i cut off from junked
Window unit. @ Ash series 125 watt choke/ ballast
Set up works ok with propper tubes, but our H0 tubes
Us bit different and need a cap reduction to reduce the lamp current a bit. Well i got combo cap to experment with to see the what value I need. Also i got sRS ballast
Well one of them came from a refrigeration case to run our HO lamps and look EU made SRS ballast. Then I imported in a Thorn SRS 40 watt ballast. I got somthing
Cooking up soon.

I realized that when Ash mentioned the circuit breaker. Thankfully I stocked up on fuses and holders when I went to Florida.

One thing I dont use is inline automotive type fuse holder those wont pass NEC.

You dont have to pass NEC for an experiment

But 12V (actually 32V rated) fuses on 240V that does not pass common sense either....

The fuse holders are the linear ones rated for 10a 250, but the experiment is only gonna draw a fraction of an amp. The package says in line fuse holder.

The fuse is there for the case when the current gets high due to a short circuit.....

Yes but would it be ok to use said holders or would I have to use a different one.

It depends on :

The distances between metal conductors of the 2 terminals (air distance & length of path on the surface on the plastic), and distances from them to Earthed metal surfaces (to which the holder is mounted etc)

The design of the fuse itself - When it blows a very big arc will flash in the place of melting wire, and the wire metal will be vaporized and settle on nearby surfaces. With that you have to check :

 - Can thie arc touch or jump into the fuse holder ? If so, much higher distances are required to make sure that the arc won't "catch on" and keep burning in the holder

 - Is the holder plastic (especally the area between teh terminals) in "plain sight" of the fuse wire ? If so, it will get coated with a thin metal coat from what came from the wire once the fuse blows. Will the distance be sufficient to not go arcing even when the plastic gets metal plated ?



12V car fuses are only meant to handle 15V or so, where there is no concern of any of those effects : 15V at couple 100's A (the available current from the car battery with the wiring resistance) is too little to really maintain an arc even when the distance is not large

They are also open on the back, exactly on the side facing the holder. The metal plating is not big concern there, first as the plating will be very little (small short circuit bang), and 12V is too low voltage to maintain any current or arcing over the expected layer of metal (which is also not very continuous)

So basically.... Unless you have something VERY different from car fuses to plug into that holder, i think it won't be even close to the requirements

It is not a car fuse it is a 10a 250v fuse holder with a fuse that shares the same rating. The holder is plastic.

I dont know of any 250V fuse that fit the same holder as a car fuse.... I mean this sort of car fuse - https://en.wikipedia.org/wiki/Fuse_(automotive)

I had 10a 32v fuses that were the same size as  250v linear fuses.

Thats better

If the holder is isolated well (good distances etc) it will hold the voltage by itself, unless it get exposed to arc or metal vapors. You have to use a fuse that is capable of handling the expected short circuit current, so it does not explode and let the arc/metal out

Surely the best thing would be to get 2 3 amp equipment fuses with suitable holders then you would be protected and not have worry weather the fuses were up to the job in the UK portable lamps are usually protected by 3 amp plugtop fuses this should be more than good enough for your experimental rig. I say 2fuses cos youve got two 110 volt lines hope you understand what I mean