dc short circuit thoughts

pod2011

What is the prospective short circuit current of a selv system?
seen this today bathroom switch wired to socket in attic ip44 leds connected via plug <transformer in plug> wired down to bath panel
..
if water finds its way into the leds and someone is in the bath....
or am i missing something here,

2core so no earth to rcd it...???
dc pscc is milliamps and milliamps can kill.
am i right in saying this is dangerous

2011

pod2011 wrote: »

What is the prospective short circuit current of a self system?

There are two limiting factors:

1) The VA of the power supply unit or transformer.
So if it outputs 10V and it is a 50VA power supply, the maximum current that can flow is 50/10 = 5 amps


2) Ohm's law, Current = Voltage / Resistance

OR

Current = Voltage / Impedance

seen this today bathroom switch wired to socket in attic ip44 leds connected via plug <transformer in plug> wired down to bath panel

Take a look at the VA of the transformer

if water finds its way into the leds and someone is in the bath....
or am i missing something here,

In theory a current can flow, but the current and the voltage have been limited to a safe levels.

2core so no earth to rcd it...???
dc pscc is milliamps and milliamps can kill.
am i right in saying this is dangerous

mA can only kill when the voltage is high enough.
Did you ever get a shock from a welder???

M cebee

dont think the short circuit current can be calculated using VA

Bruthal

M cebee wrote: »

dont think the short circuit current can be calculated using VA

Indeed, the VA would be the continous rating. On wire wound traffos there is a saturation point above the rated VA, electronic ones would have a limit above the VA also, where they fail or cut out if they have electronic protection.

Bruthal

To say 50mA at 230v can kill, but 50mA at 12v cant is not really that accurate. The 50mA can kill, what voltage causes it to flow does not matter. But 12v wont cause it as it just cant drive 50mA through a person as their resistance requires voltages up towards 200v to cause 50mA to flow.

A person is like 3 resistors in series, the 2 points of contact which can vary hugely, and their internal resistance which is failry stable. So to get 50mA flowing through the internal resistance might require 230v with both hands making light contact with a live and neutral/earth .

But both hands wet, and a good grip of both contacts might only need 150v (example) to get 50ma flowing.

If both scenarios above give a 50mA shock, both shocks perceived will be the same, it makes no real difference that one was 50mA from a 230v shock, and the other was 50mA from 150v, the internal resistance will still have the same volt drop across it to have 50mA flowing, its the resistance of the contact points that varied.

Selv certaily cant ever have enough mA flowing to perceive in a person, and being the mad electrician type, i tested it. 12v lipo battery, and ammeter in series with myself gave 100 micro amps with a good grip on the contact points. With wet hands it gave 500 micro amps with a similar grip.

So compare this to the 50ma it is said is around the lethal level.
50mA = 50,000 micro amps
12 volt test with dry hands = 100 micro amps (0.1 mA)
12 volts test with wet hands = 500 micro amps (0.5mA)

So the current flow through a person from a 12 volt battery is never going to be of any danger, but it can clearly be seen how much more dangerous wet hands would be on 230v shocks than dry hands.

Also it can be seen how widely the current varied in the tests. And the internal resistance and voltage remained constant, only the contact point resistances varied. So 50mA through the body can kill, and it does not matter what external voltage causes this 50mA, a 50mA shock will still be percieved the same in each case. But it wont ever happen from selv, likely needing well above the 100v mark.

2011

M cebee wrote: »

dont think the short circuit current can be calculated using VA

I did not say that it did. Take another look at my post my post, I said that it was a "limiting factor"

2011

robbie7730 wrote: »

But 12v wont cause it as it just cant drive 50mA through a person as their resistance requires voltages up towards 200v to cause 50mA to flow.

+1
As, I said, Ohm's law

Bruthal

It did say for a 50va 10v traffo that the max current would be 5A. Maybe it shoud say the max continous current.

M cebee

quote:
"1) The VA of the power supply unit or transformer.
So if it outputs 10V and it is a 50VA power supply, the maximum current that can flow is 50/10 = 5 amps"

if you have a trafo for example -the max current that can flow isn't determined by the VA:)

2011

M cebee wrote: »

quote:
"1) The VA of the power supply unit or transformer.
So if it outputs 10V and it is a 50VA power supply, the maximum current that can flow is 50/10 = 5 amps"

if you have a trafo for example -the max current isn't limited by the VA:)

I still stand by the above.
What I said is that if the output is 10 volts for a given circuit fed from a transformer (or PSU) which is rated 50VA, then it is not possible for more than 5 amps to flow.

I did not say that 5 amps would flow. I could only work out what will flow by knowing the resistance if it is DC or the impedance if it is AC.

Bruthal

2011 wrote: »

I still stand by the above.
What I am said is that if the output is 10 volts for a given circuit fed from a transformer (or PSU) which is rated 50VA, then it is not possible for more than 5 amps to flow.

I did not say that 5 amps would flow. I could only work out what will flow by knowing the resistance if it is DC or the impedance if it is AC.

M cebee is saying more than 5 amps will flow if more than 50watt load is put on it, and id say so myself too. The VA would be the continous rating rather than the saturation point.

2011

M cebee wrote: »

quote: if you have a trafo for example -the max current that can flow isn't determined by the VA:)

Correct.
However if you know what the voltage is for given circuit and you know the VA of the transformer suppling the circuit, then you can work out the maximum current possible. You will need more information to calculate the actual current that will flow.

M cebee

well ya- i thought we were talking short-circuit here

not continuous ratings

2011

M cebee wrote: »

well ya- i thought we were talking short-circuit here

If your point is that under short circuit conditions there will be an initial "spike" in the current before it settles down, I agree completely with you.

My main point in reply to your post was that I was not suggesting that the VA alone could be used to calculate the current flowing.

I tried to address the OP's question without getting too technical, that is why I talked of "limiting factors" rather than giving specifics.

M cebee

no worries

Bruthal

A 50va transformer can supply more than its rated 50va though. A short circuit will cause a transformer to supply up to its saturation point, more appropriate for wire wound ones, but the 5 amps is a continous rating, it could supply 6 amps if a 60 watt 10v bulb was put in, unless it is electronically limited to the 5 amps exactly. Thats not getting very tecnical.

M cebee

is the short circuit current limited to the rated current on those electronic jobbies ?

i know they have short-circuit protection

Bruthal

M cebee wrote: »

is the short circuit current limited to the rated current on those electronic jobbies ?

i know they have short-circuit protection

It certainly can be done alright, no doubt about that, but it would seem unlikely to be right at the VA rating. Maybe its time for more experimenting as i have a few 12v electronic traffos here for powering battery chargers.

For wire wound ones, the VA would be lower than the actual short circuit current, which would be limited to the saturation point of the windings.

pod2011

thanks for the replies
its a 12v output 5w max
so i=p/v 5w/12 = .416amps = .000416ma which should cut out the trafo

no where near the 50ma needed

2011

No,

0.416 A = 416 mA

Bruthal

pod2011 wrote: »

thanks for the replies
its a 12v output 5w max
so i=p/v 5w/12 = .416amps = .000416ma which should cut out the trafo

no where near the 50ma needed

5 watt output seems very small, but anyway,

Its 416mA as 2011 said, which is near 10 times what would be a fatal current through a person.


However, if a 5 watt bulb is in use, it will have a resistance of 12v/0.416A=28 ohms.

A person could be 2000 to 4000 ohms internal resistance but this can vary a lot. But just take 3000 ohms, so the current from 12v would be 12/2000=0.006 amps or 6 mili amps.

In the tests in post #5 the highest i got with 12 volts was 0.0005 amps or 0.5mA which shows a total resistance (internal + skin) of 24000 ohms.


A car battery will supply about 85 amps to a 1kw 12v starter motor, but if you go and touch the battery terminals at the same time, you will still only have less than 1mA flow through you, while the 85 amps flows to the starter motor on starting the car. This is because you are many thousands of ohms, while the starter motor is 12v/85A=0.14 ohms.


In a slight tangent off topic, in reality, the starter motor is actually a good bit lower than this 0.14 ohms, because it actually generates a back emf as it speeds up from stopped, and this limits the current to 85 amps full load, which means the more the motor is slowed by a heavier load, the higher a current it will take as the back emf generated is lower.

So with no load, it will speed up to a level where the back emf almost equals the supply voltage so the unloaded motor will take the least current, and the difference between the two (back emf & supply V) being sort of like the difference between the synchronous and rotor speed of an AC induction motor with no load on it.

In the dc motor it cant quite reach the speed where the back emf = supply voltage, as there would now be no current drawn by it, and with the induction one, synchronous speed cant be reached as no current would then be induced in the rotor as it has to be moving slower than the magnetic field is changing for this to happen.

pod2011

Thanks for clearing that up , i was right before i edited my post with the 416ma but couldnt get head around it being more than 50ma
its the high resistance of the body that and the low voltage.

Thanks again

Bruthal

pod2011 wrote: »

Thanks for clearing that up , i was right before i edited my post with the 416ma but couldnt get head around it being more than 50ma
its the high resistance of the body that and the low voltage.

Thanks again

Yes thats it, the 5 watt bulb takes 416mA. A 100 watt bulb also takes very close to that. But its resistance is 530 ohms compared to the 12 volt lamps 28 ohms. So it takes 230v to drive 430 mA through a 100w 230v bulb. Its worth noting the 100w bulbs resistance is far lower than 530 ohms when its cool, probably about 40 ohms or even less, but when its white hot, it rapidly increases.

This means an incandescent bulb has a high surge or starting current, which can cause problems if there were large numbers of them switchen on together. This surge can actually be seen if you look at an esb meter and have someone switch on a 100w bulb, the meter will briefly start fast and slow almost immediately.

If you connect the 12v to the 530 ohm 100w 230v bulb, in theory it would be dissipating 0.27 watts, but in reality the 230v bulb would not heat up enough to increase its low resistance so it would be about 3 or 4 watts, compared to the 100 watts with 230v connected to it.