Back when I was chipping I did the Test n tag course in 04. 1 day course and a testing machine and I could test most stuff. Most of testing process involves basic checking of leads and connections, only then do you plug it in and check it electrically. Never had something fail on the box and pass the safety switch. Safety switch are mandatory , tagging caravans is money raising

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Paddles B'mere said..
It's the increase in current that does the damage Mark, this is because of the reduced voltage. It'll still run fine but in extreme cases the increased current can cook the windings and the motor carks it quicker.

V=IR (and your compressor motor is the R) so if V drops then I goes up because R is constant.

Cracking good electricity jokes there too lads

Good...but can you dumb it down a little more?

Nah, it's a can of worms Kato. A licensed electrical contractor has to take responsibility for the LV installation in a house but no one has to take responsibility for the LV install in a van or motorhome and it's every bit as dangerous.

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Paddles B'mere said..
It's the increase in current that does the damage Mark, this is because of the reduced voltage. It'll still run fine but in extreme cases the increased current can cook the windings and the motor carks it quicker.

V=IR (and your compressor motor is the R) so if V drops then I goes up because R is constant.

Cracking good electricity jokes there too lads

Hey Paddles, I think you might want to balance that equation a bit better.... Aren't you an accountant?

E = I.R means that if R is a constant, and I drops, so does E and vice versa.

If you are an electronics person then.... ummm....

You're right FN, I've had a brain fart and it's the extension of Ohm's law for power that is P=VI that makes the current go up when volts drop. I'm forgetting the engineering as I learn the accounting

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Paddles B'mere said..
You're right FN, I've had a brain fart and it's the extension of Ohm's law for power that is P=VI that makes the current go up when volts drop. I'm forgetting the engineering as I learn the accounting

Hopefully we use the numbers the same way

Oh, "creative accounting..." I get it. - What do you want the number to be?

5%

Fine. My question still stands, and I'll direct it at both FN and Paddles. Could you dumb it down a bit?

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FormulaNova said..

Paddles B'mere said..
You're right FN, I've had a brain fart and it's the extension of Ohm's law for power that is P=VI that makes the current go up when volts drop. I'm forgetting the engineering as I learn the accounting

Hopefully we use the numbers the same way

Oh, "creative accounting..." I get it. - What do you want the number to be?

Your on the money paddles. At start up, the winding in an electric motor have bugger all impedance until the rotor can create a back EMF. Low voltage results in a higher and longer start current which can result in overheating of the windings and damage to the start gear (switch).

On start up, the equation V = IR (I = V/R) is only linear for resistive loads like heating elements or filament light bulbs.

Low voltage = high current for a constant power so its the heat from the increased current that kills the windings on an unprotected motor like a power tool. Bigger motors will have thermal protection that will cut the supply if the motor gets too hot. For a larger loads on a long lead you should try to use the biggest cable/lead you can find to carry larger currents whilst minimising the voltage drop on the cable. Motors at start have high inrush currents (I've seen up to 13x full load current) and the voltage drop can be horrendous. There are some other considerations but thats the long and the short of it.

Edit: and yes, DC current can be very effing nasty especially if it's from a supply that can pump out a large current like a car battery

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Crusoe said..
On start up, the equation V = IR (I = V/R) is only linear for resistive loads like heating elements or filament light bulbs.

Nothing linear about a filament light bulb. Go and measure the resistance of a filament before it's turned on and then calculate P=V^2/R. You'll find the answer is not the same as the light bulb rating because the resistance changes significantly when the filament reaches whopping high temperatures.

Right so for my compressor I need to get the shortest lead I can use, and try to make it a fat 15A one.

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Mark _australia said..
Right so for my compressor I need to get the shortest lead I can use, and try to make it a fat 15A one.

That was my end solution. two 15amp leads with 10amp plugs. one at 15metres, the other 10metres, just use the shortest one I can on the day. Haven't had a problem since

Cool my plan was similar - 15A cord and file down earth pin so a cheap way to get a fat cord.

Two leads in parallel with a double adapter at each end?

Call me Mark

I was thinking about similar to run 2 x 10A from different sockets so as to run a much larger compressor. Sparky mate suggested it but said leads need to be exact same lengths and its dodgy AF..... so then I thought if house burns down there is no insurance and so on. Not worth the risk....
But modified 15A just to get a thick cord sounds great

"Running two leads is dodgy", he says to himself, as he files down the pins of a 15A plug so he can put it into a 10A socket.

Nothing dodgy about running two leads, if they aren't exact length, who cares? So, they share current 66/33 instead of 50/50, you still get significant reduction of voltage drop. Running out of two sockets won't really achieve anything if they are on the same circuit anyway. And if they are on different circuits, be careful if you have 3-ph power!

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Harrow said..
"Running two leads is dodgy", he says to himself, as he files down the pins of a 15A plug so he can put it into a 10A socket.

Nothing dodgy about running two leads, if they aren't exact length, who cares? So, they share current 66/33 instead of 50/50, you still get significant reduction of voltage drop. Running out of two sockets won't really achieve anything if they are on the same circuit anyway. And if they are on different circuits, be careful if you have 3-ph power!

The dodgy bit is how you are connecting them together towards the load end. A lot of 'potential' for having an exposed plug that is live from the other cable! Just like all these people that attach a plug onto both ends of a lead to power a caravan from an internal socket.

If you brought both leads into one junction box at the load end, what have you ended up with? You can't use different circuits as it could be different phases(!) or bypasses the protection on the circuits as they could be live even when supposedly isolated. (Edit - I just re-read Harrows reply on this, and yes, two phases shorted together would be interesting!)

This means that you are running two power points from the same circuit, which gains you next to nothing. If your appliance is still 10amp, get a better extension cord. If its rated at 15amp, get a 15amp GPO and extension cable.

I think the 15 amp extension with 10amp plugs and sockets fitted is a better idea. I bought some heavy duty flex from Bunnings to do something similar but its sometimes a bit harder to get thicker gauge cable to fit into 10amp plugs and sockets.

When you look at the cable thickness in some extension leads rated for 10amps, it makes you wonder how long you can get 10amps out of it before the lead gets warm.

Going completely off-topic, there are adapters to run caravans with 15amp feeds off of a 10amp socket, and they do it legitimately by having an 10amp circuit breaker to limit the max current.

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FormulaNova said..The dodgy bit is how you are connecting them together towards the load end. A lot of 'potential' for having an exposed plug that is live from the other cable! Just like all these people that attach a plug onto both ends of a lead to power a caravan from an internal socket.

Excellent point. That alone is good enough reason not to use two leads.

And as I've attested previously, using paper clips to connect a 15A plug to a 10A socket produces an amusing spectacle at night.

A plug at each end of a lead?!

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Harrow said..
"Running two leads is dodgy", he says to himself, as he files down the pins of a 15A plug so he can put it into a 10A socket.

Well if you can't buy a fat extension cord.....

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Mark _australia said..
Well if you can't buy a fat extension cord.....

If the plug on your motor is only a 10A one, then fair enough.

Curiosity got the better of me so I measured the resistance of 15 metres of 10 amp extension lead. 0.1 ohms according to my multimeter! (plus or minus 0.1 is the limit of its resolution, but anyway, not much.) . That's 0.2 ohms there and back so should only lose about 2 volts, (maybe 4 volts) at 10 amps. What's everybody else's multimeter say?

So I thought I would rush downstairs and check the resistance on a few leads, then I realized my beer is still half full so I will wait until it is empty, beer is downstairs too.

PS You can buy leads with heavy cord and 10 amp plugs, but they are not $19.95 for 20m

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Ian K said..
Curiosity got the better of me so I measured the resistance of 15 metres of 10 amp extension lead. 0.1 ohms according to my multimeter! (plus or minus 0.1 is the limit of its resolution, but anyway, not much.) . That's 0.2 ohms there and back so should only lose about 2 volts, (maybe 4 volts) at 10 amps. What's everybody else's multimeter say?

I just tested one here and it seems to be around 0.5 to 0.4 ohms each way. Some generic extension lead of 10m.

So, assuming a total of 1 ohm, its a 10 volt drop at 10amps. The mains can vary that much anyway, so is it going to matter?

I guess the answer is to get the best quality extension lead you can, but no one rates them other than '10amp' which means nothing really.

I only have 1 proper lead to test and it is an old (5-10 yrs old) 20 meter lead I measured 0.61 ohms

The best bet; if you really want to protect a valuable piece of kit like a good compressor that is permanently positioned; is to just get a sparky to install another outlet near the machine. He or she will probably run a length of 2.5 or 4sq.mm 2C+E to the location depending on the install and then the problem goes away. Most cheap power leads only have quite small cable size.

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Paddles B'mere said..
The best bet; if you really want to protect a valuable piece of kit like a good compressor that is permanently positioned; is to just get a sparky to install another outlet near the machine. He or she will probably run a length of 2.5 or 4sq.mm 2C+E to the location depending on the install and then the problem goes away. Most cheap power leads only have quite small cable size.

Usually TPS 2.5mm as it's good for 20A. 4mm is used for 32A.

1mm FLEX is good for 10A.

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Greenroom said..
Usually 2.5mm as it's good for 20A. 4mm is used for 32A.

Out of curiosity, I did some arc welding at 180A with 0.5mm leads once. It worked surprisingly well...for a short while. Not a lot of insulation left on the wires when I was done.

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Mark _australia said..
Cool my plan was similar - 15A cord and file down earth pin so a cheap way to get a fat cord.

how do you narrow the slot for the earth at the other end.....