RCDs and cable dimmensions

[lael]

I bought a couple of power boards with RCDs on them. They definitely work. I was hoping that the power board RCD would mean the main board wouldn't switch, but the main board had triggered as well each time so far.

 

[jibba02]

As long as the rcd has an Australian standards sticker on it. It will be fine

 

[Camo6]

Or a Swedish standards sticker?

 

[spog]

I bought a couple of power boards with RCDs on them. They definitely work. I was hoping that the power board RCD would mean the main board wouldn't switch, but the main board had triggered as well each time so far.

Double up,I don't see any harm in that.

 

[lael]

Yeah, no harm, but curious why it triggers both. I was hoping that the powerboard one would stop the current fast enough that the main switch would continue operating.

 

[TheWiggman]

I'd say because both would detect the earth leakage. If you're talking 30ms on a solid state device it would be on its way to tripping before the downstream one tripped at a guess.

 

[lael]

yeah, that is what I was guessing, or that the current isn't immediately clamped, and so trips the first one, which clamps/goes off, but not before the differential travels past and trips the second

 

[jimmy_jangles]

Cabling is designed with a working temperature of 60c-110c when used in 40c ambient temperature, 1.5mm of a reputable brand can take 16a.

Are you F^&^%$G serious

I hope those cables are very short

You need to spend a lot more years in the game my friend

Great way to reply to people that don't know what they are talking about and are after advice. I assume by the use of the bridge rectifier in your signature you would like people to think you are superior in regards to your electrical knowledge? Maybe just share that knowledge next time instead of pointing out peoples mistakes.


So i've had my rant, now what old mate means is that when you increase a cable's length, you increase the resistance in the cable, which in turn decreases the amount of current that it can carry, If you wanted to search for the standards, it's AS 3008 that we use here ( it may be a diff standard over there) and check out table 10 that shows the capacity of twin and earth cables. Keep in mind there are derating factors that apply too, so it's not just the number that the table says, rather that is the number you plug into an equation to give you your capacity. I would be using 2.5mm cable for peace of mind, however the lads were right that most extension leads are made from 1.5mm cable.

The thing you need to keep in mind about that is that these are made with 10amp plugs on the ends of them. Meaning they are really only rated to 10amps. If they were rated higher, for example 15 amps, they would have a different plug on them that does not fit into a 10amp General power outlet. The difference is the size of the earth pin is larger so they can't be plugged into lower rated outlets.

In regards to why the rcd at the boards trip before the portable ones, it would be a timing issue, i'd be willing to bet that the rcd installed in the board is set to trip slightly quicker than the portable one, or may be calibrated a bit better and in turn detect the leakage quicker. Would love to be told i was wrong on this if anyone else has had experience with the prob and knows a bit more about it.

Dave if you have any other questions that you wanted to ask before your electrician comes, feel free to shoot me an email [email protected]

anyway, enough yabbing from me, my glass is empty so it's time to go and refill!

 

[dave_h]

Thanks Jimmy_jangles, should be good to go.

I will upgrade the cables, I think the plugs on my extension leads are 16amp ones but when I get new ones I will make sure they are.

Dave

 

[GibboQLD]

when you increase a cable's length, you increase the resistance in the cable, which in turn decreases the amount of current that it can carry

Don't you mean voltage?

All things being equal (except cable length in this case), it can carry the same amount of current, but the voltage drop increases with cable length.

 

[jimmy_jangles]

Don't you mean voltage?

All things being equal (except cable length in this case), it can carry the same amount of current, but the voltage drop increases with cable length.

Yes you do get voltage drop, but your supply voltage will not change, so the increase in cable length will decrease the current carrying capacity at that supply voltage

that makes sense to me, but i could be explaining it wrong

 

[Pokey]

Yes you do get voltage drop, but your supply voltage will not change, so the increase in cable length will decrease the current carrying capacity at that supply voltage

that makes sense to me, but i could be explaining it wrong

You're combining different issues. Current carrying capacity is based on thermal limits of the cable, there are specified limits on voltage drop that also need to be met, 5% voltage drop I think from point of supply to end of the circuit.

 

[klangers]

Just clearing up a bit of confusion here:

RCDs - Residual Current Devices. These detect a current imbalance between the terminals (active and neutral). If there is an imbalance this means that some current is escaping through another path (eg your body). The RCD trips when an imbalance of (usually) 30mA is detected. 30mA flowing through your heart is enough to kill you. Designed to protect you.

CBs - Circuit Breakers. These simply trip when the rated current is exceeded. Designed to protect equipment.

Often, these two functions are combined together into one device. Often they're not too, so be sure what you're doing.

Electricity is like water*. Try pumping heaps of water through a small pipe - you need lots of pressure. Try pushing heaps of current through a small cable - you need lots of voltage.

*for layman comparison purposes

 

[sponge]

Afer re-reading what I had posted, I'll happily put my hand up for biggest brain fart of 2016. Klangers is completely correct (below)!

This is why I shouldn't jump straight back into work after a month off..

 

[klangers]

Voltage drop at final load (heating element for example) will increase current draw. Ie, 220V at 3.2kW element draws 14.5A compared to 13.3A at 240V. The smaller the cable size, the larger the voltage drop over the same distance and current..

Not quite. The resistance of the element is constant, and so is the cable (since we're talking a purely resistive load, rather than inductive or capacitive and a constant frequency, and assuming negligible temperature rise). So at your source you have (ideally) 240 V. The 240V is applied to the cable and element, which induces a current to flow. The higher the resistance (longer cable, smaller element), the lower the current that flows. The lower current flow = lower power.

So, if a 3.2 kW element draws 13.33 A at 240 V (at the element), it is an 18 Ohm element (V=IR; R=V/I; R=240/13.33). Assuming your cable has zero resistance for this exercise, if only 220 V makes it to your element then it will only draw 12.22 A, effectively de-rating your element to 2,689 W.

 

[sponge]

Completely agree with you mate and have edited my post.

By far one of my silliest mistakes.. Carry on.