Is a 45A circuit suitable for a 17.1kw electric cooker?

[FredW]

So are you expecting very high protective conductor currents from this cooker? To the extent that the cpc normally considered adequate in 10mm² T/E would not be sufficient?

To answer you shortly BAS, yes and no!

*this is a bit off the topic of this tread, but I love your comments, so hopefully you will take the bate*

I belive the BS wiring standards are now lagging far behind rest of Europe, i.e. DIN standards. This goes for earthing, no "neutral"- MCBs / "common neutrals" (i.e. the challenges with borrowed neutral) and the lovely ring circuits! (copper isn't that expensive post-1930s so in my opinion rings should be illegal, and MCBs with N+L switching isn't much more expensive than a single L MCB any longer, nor do they take up more space in the CU) It is of course absolutely fine in UK and Ireland (we follow you guys) to use T&E's earth lead for earthing appliances (in most cases, even this cooker installation), but personally I would like to see some changes to the BS. I would like the same sqmms for the "lives" (L+N) as for the earth, why- because you don't want to increase the resistance of electrons flowing from L or N to PE by reducing the earths sqmm relatively to L+Ns sqmm. I am sure you have seen that in some circumstances T&E 1.5mm2 cores come with a 1.0mm2 earth core. (This you could find in cable sold by some of the beloved sheds, and these cables again in my opinion should never been BS certified).

Now lets get back to what kills people electrical wise. It is of course not voltage (in low voltage systems anyway, contradicting some of the comments posted on other treads in this forum), but current. A 45A appliance as the cocker, will theoretically draw up to 45A before the 45A MCB will say thats enough, and even then, you will find your N to now be "L" and without a MCB protection at all. If it was a short in this cooker, and let say, my body made a better connection path to earth than the PE core (I might have been wet after a shower, I might have been without slippers, and the kitchen might have been on ground floor) well, then I would probably have been on my way to to join the harp gang (AC>100mA-or DC>300mA through the heart will get me there), and I wouldn't have been to much happy with the installed cable. Other words, I would treat all high kW/ VA rated fixtures with respect, and install earth cores same size as L and N or larger to minimize the resistance in the cable itself. Also of course RCDs and SPDs should be used, in all installations.

Ps. I would also like to see proper earth rings to be used around residential homes (and sheds) and not a silly copper stick drilled to the ground Ds.

 

[Space cat]

If it was a short in this cooker, and let say, my body made a better connection path to earth than the PE core ---

If you body makes a better connection to earth than the (properly connected) PE core, you must be some kind of cyborg.

If you draw the circuit of the shorted cooker, you'll see that the worst case scenario is a dead short from the incoming live terminal to earthed metal. The wires in the cable form a potential divider across the supply so what voltage exists on the cooker itself? If the earth core is the same size as the supply core, the answer is one half of supply voltage at the CU. (This will have dropped somewhat because of the enormous current your cooker is drawing.) You can forget the neutral wire because that has the resistance of the cooker in series with it.

This is the voltage across your body. The current that will flow through you depends upon your resistance, including the path from your feet back to true ground. If, as you suggest, you are dripping wet, that will be lower than usual - but not nearly as low as it would be if your were holding an earthed water tap with your other hand! The resulting current could prove fatal.

Now let's make the earth core smaller. Result: The voltage on the cooker's metalwork will be higher. This is not good for you. Moreover, because the fault current will be lower, the breaker will take longer to trip. This is also not good for you.

Yes, it would be better to have a bigger earth core. It would be better still to have a very big earth core linking the cooker directly to the tap - or to any other bit of earthed metal you could reach. If you want to go to extremes, you can have an earthing grid in your kitchen floor and attach all of your appliances to it with nice thick copper bars - but that might just be overdoing it!

 

[ban-all-sheds]

*this is a bit off the topic of this tread,

Then why did you post it in this thread?

I belive the BS wiring standards are now lagging far behind rest of Europe, i.e. DIN standards. This goes for earthing, no "neutral"- MCBs / "common neutrals" (i.e. the challenges with borrowed neutral) and the lovely ring circuits! (copper isn't that expensive post-1930s so in my opinion rings should be illegal, and MCBs with N+L switching isn't much more expensive than a single L MCB any longer, nor do they take up more space in the CU) It is of course absolutely fine in UK and Ireland (we follow you guys) to use T&E's earth lead for earthing appliances (in most cases, even this cooker installation), but personally I would like to see some changes to the BS. I would like the same sqmms for the "lives" (L+N) as for the earth, why- because you don't want to increase the resistance of electrons flowing from L or N to PE by reducing the earths sqmm relatively to L+Ns sqmm. I am sure you have seen that in some circumstances T&E 1.5mm2 cores come with a 1.0mm2 earth core. (This you could find in cable sold by some of the beloved sheds, and these cables again in my opinion should never been BS certified).

Fine, and those are all valid topics for debate, but when a DIYer posts questions about whether his electrician is doing OK don't post your opinions and wishes for changes as a factual answer of what should be done.

Now lets get back to what kills people electrical wise. It is of course not voltage (in low voltage systems anyway, contradicting some of the comments posted on other treads in this forum), but current.

But no current will flow unless there is a voltage difference.

A 45A appliance as the cocker, will theoretically draw up to 45A before the 45A MCB will say thats enough,

A lot more than that, in fact - read this: //www.diynot.com/forums/viewtopic.php?p=76467#76467

and even then, you will find your N to now be "L" and without a MCB protection at all.

¿Que?

If it was a short in this cooker, and let say, my body made a better connection path to earth than the PE core (I might have been wet after a shower, I might have been without slippers, and the kitchen might have been on ground floor)

Make whatever assumptions you like for wetness, footwear and flooring.

Then show how your body is going to provide a path to earth of less than 20-30mΩ

Other words, I would treat all high kW/ VA rated fixtures with respect, and install earth cores same size as L and N or larger to minimize the resistance in the cable itself.

Or larger? FFS.

Anyway - please explain how a fault between line and an earthed exposed conductive part of an appliance is more of a hazard if it's a high power appliance.

Please also explain how the difference in resistance of a 10mm² cpc compared to a 6mm² one will result in a significant difference in touch voltage on the exposed conductive part, and/or a significant difference in time before the MCB operates.

Feel free to show valid calculations.

 

[Space cat]

Please also explain how the difference in resistance of a 10mm² cpc compared to a 6mm² one will result in a significant difference in touch voltage on the exposed conductive part, and/or a significant difference in time before the MCB operates.

I'm not taking any sides here but ---

1) Live = earth = 10 sq mm. Both cores have the same resistance and so the touch voltage is one half the supply voltage.

2) Live = 10 sq mm, earth = 6 sq mm. Ratio of resistances is 3 to 5 so touch voltage is 0.625 x supply voltage. That, in very rough figures, is an extra 30 volts.

Now an extra 30V isn't huge but it's not insignificant either. There's a flaw in the semantics here. Most people would consider 'significant' to mean something big while 'insignificant' means something tiny. What about all the stuff which is neither?

The loop resistance of the two wires is one third bigger in the second case so the current will be 25% lower. Will this cause a 'significant' increase in tripping time?

 

[ban-all-sheds]

That, in very rough figures, is an extra 30 volts.

So 145V instead of 115V.

You'd be happy to put yourself between 115V and earth, would you?

Now an extra 30V isn't huge but it's not insignificant either. There's a flaw in the semantics here. Most people would consider 'significant' to mean something big while 'insignificant' means something tiny. What about all the stuff which is neither?

I meant "significant" as in important.

Presumably your answer to my question above is "no", in which case you don't consider the difference to be significant.

The loop resistance of the two wires is one third bigger in the second case so the current will be 25% lower. Will this cause a 'significant' increase in tripping time?

OK - let's assume a 10m cable, a B45 and a Ze of 0.35Ω

R1+R2 for a 10mm²/10mm² cable is 0.0366Ω

R1+R2 for a 10mm²/6mm² cable is 0.0491Ω

So at 230V the fault current with a 10mm²/10mm² cable is 595A.

With a 10mm²/6mm² cable it's 576A.

Take a look at the time-current curve for a B45 and see if there's a significant difference in the tripping time between those two currents.

 

[Spark123]

OK - let's assume a 10m cable, a B45 and a Ze of 0.35Ω

R1+R2 for a 10mm²/10mm² cable is 0.0366Ω

R1+R2 for a 10mm²/6mm² cable is 0.0491Ω

So at 230V the fault current with a 10mm²/10mm² cable is 595A.

With a 10mm²/6mm² cable it's 576A.

It depends on the temperature of the cable when the fault occurs - if it is at its max operating temperature when the fault occurs then the resistance will be 20% higher thus the current will be lower.
A 1m long circuit isn't going to be much use!
A B45 MCB is going to disconnect in 0.1s as long as the fault current exceeds 225A.

 

[Space cat]

You'd be happy to put yourself between 115V and earth, would you?

No!

Presumably your answer to my question above is "no", in which case you don't consider the difference to be significant.

I'm with you now. This is like saying "Do you care whether you fall off a 115 ft building or the one next door which is 30 ft higher?" When you put it that way, the difference isn't significant. On the other hand, the difference between 115V and 145V might be very significant if the 115V doesn't quite kill me - and no, I'm not going to put this to the test!

It comes down to cost. How much extra are we prepared to pay for that extra safety margin? Around here we have a different hazard, namely radiation, and all our safety rules are based on a principle known as ALARP. This stands for "As Low As Reasonably Practicable". It used to be ALARA, where the final A stood for "Achievable" but that was flawed because you can always get the dose lower. How much concrete are you prepared to pay for? The reality is that ALARA still applies - but it's "As low As Reasonably Affordable".

 

[electronicsuk]

Anyway - please explain how a fault between line and an earthed exposed conductive part of an appliance is more of a hazard if it's a high power appliance.

Hit the nail on the head there. Regardless of whether you end up on the wrong end of a 200A submain protected by BS88 fuses or the load side of an FCU with 3A BS1362, neither fuse is likely to offer you any protection in terms of electric shock, and the same goes for an MCB.

There are various figures floating around with regard to the resistance of the human body under various circumstances. However, some of the worst case scenarios I've been able to find information on (i.e. feet in water at ground potential, live conductor in hand) quote body resistance down to anything as low as 100 Ohms. Even then, 230v / 100 Ohms = 2.3 Amps. Granted that the affects of the electric current may cause your body's resistance to decrease over the duration of the shock, I think it's safe to say that by the time any overcurrent protective device you'll find in common use in a domestic situation operates, you'll not be around to worry or care about it.

Oh, and before anyone tries going down the path of "well, R1+R2 and PSCC will be a lot higher on the submain...", well, that may be true - but it's not even worth considering when you consider that the bulk of the potential is going to be across the body. That is, unless you have a supply impedance getting on for close to the impedance of the human body, which I doubt

Sorry to go OT as I appreciate that the original (also OT!) discussion was related to the potential between a metal appliance with a L-E fault and true ground, and that's not really what I was getting at. I just get a bit sick of hearing "It's the current wat' kills ya!". It's true, but often misused by people who somehow seem to think that the capacity of the supply is going to have any real effect on the severity of the shock. Guess that's what we have RCDs for!

 

[JonoPashley]

MCBs are for Overload and Overcurrent Protection. RCDs are to provide shock protection. So the earth being 10mm2 argument is pretty invalid.

 

[aptsys]

I just get a bit sick of hearing "It's the current wat' kills ya!". It's true, but often misused by people who somehow seem to think that the capacity of the supply is going to have any real effect on the severity of the shock. Guess that's what we have RCDs for!

That statement annoys me no end also. Energy (J), is a more accurate unit to measure the likelihood of fatality.

 

[ColJack]

current is energy flow per second right?

so if I have 1,000,000J of energy flow through me for 1/1,000,000,000,000th of a second, it's not going to do as much damage as 1,000J of energy flowing through me for 10 seconds...

so therefore current is correct..

however, current is not a "likleyhood" it's an "actuality"..
until it's flowing then it doesn't exist....
the "likelyhood" is the voltage, which is the measurement of potential energy that can flow..

so voltage is an indication of "likelyhood of fatality", whereas the current that flows when you complete the circuit is the actual thing that causes the damage that leads to fatality..

 

[FredW]

Agree with ColJack.

I=Q/T [A=J/s]

Good article about effects of electricity on the human body: http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

 

[Space cat]

current is energy flow per second right?

Not quite. Current is CHARGE flow per second and energy is charge multiplied by voltage but the general idea is correct. Time matters.

When I was at school, the Van der graff generator was a standard item in most physics labs. It put out about 300 kV and we all queued up to draw sparks off it. I measured my own hand-to-hand resistance at the time and it was about 50 k-ohms. (I also learnt how to vary it to order but that's not important right now.)

A simple calculation shows that the thing was pushing 6 amps through my body but I survived. We all did. Why? Because that metal sphere held a charge of about 3 micro Coulombs and less than half a Joule of energy. That's just enough to make you jump.

voltage is an indication of "likelyhood of fatality", whereas the current that flows when you complete the circuit is the actual thing that causes the damage that leads to fatality.

If you substitute "charge" for "current" then you are correct.

 

[ban-all-sheds]

let's assume a 10m cable

1m long circuit isn't going to be much use!

 

[Spark123]

oops - missed that they were x10.
Still need to compensate for temperature rise tho.