Your Favourite Electrical Engineering College Course Question

How about this for a question

You attend at an average UK 3 bed semi detached house built 1920/30's and they have 3 Balcony Solar PV panels attached using uk plug bs1363 to the ring final.  How do you proceed?  What would you check?  What would you advise the homeowner/duty holder?

How about this for a question

You attend at an average UK 3 bed semi detached house built 1920/30's and they have 3 Balcony Solar PV panels attached using uk plug bs1363 to the ring final.  How do you proceed?  What would you check?  What would you advise the homeowner/duty holder?

built 1920/30's

Well if it is as-built, you might start by refusing the cup of tea, as it probably has lead pipes, and that may also save you needing to use the outside loo... It's not the age of the property, it's what has been done to it since, and of course, why you have been asked to attend in the first place.

But we all know I hope that "real world" questions confuse both candidates and examiners and are to be avoided.  ;-) 

I do remember a nasty exam one where two capacitors at different starting voltages are connected by a switch, and to calculate the final voltage. The trick is to realize that charge (C.V)on either side of the insulators is redistributed but conserved, but energy (1/2 CV^2) is not - in fact it never is, the only question is where it goes, and there is always a loss somewhere but not one actually shown in the question.. 

There is a related variation question about switching inductors -  where the momentum like quantity L.I is conserved but 1/2 LI^2 energy is not.

Actually anyone who has stared sadly at the burnt points on a car with conventional ignition can testify where that energy goes to, or indeed the larger version when the older London underground trains change winding configuration as they pick up speed and there is a loud "pock" and blue flash and in that case it feels like actual momentum is not always conserved either....

Mike.

t an average UK 3 bed semi detached house built 1920/30's

Well if it was still a bit old fashioned and had its sockets on rewireable fuses and 7/029 cable you could probably add 800W of solar without degrading the safety of the original installation at all - there being no RCDs to worry about and the slightly thicker cable having enough extra capacity to accommodate 2/3rds of 800W without any chance of overloading.(Presuming the inverter does its stuff and disconnects promptly on grid loss)

You attend at... What would you advise the homeowner/duty holder?

probably depends on what capacity you're there...

   - Andy.

Maybe correct for 1 balcony solar but my question was for 3

mapj1 said:

I do remember a nasty exam one where two capacitors at different starting voltages are connected by a switch, and to calculate the final voltage. The trick is to realize that charge (C.V)on either side of the insulators is redistributed but conserved, but energy (1/2 CV^2) is not - in fact it never is, the only question is where it goes, and there is always a loss somewhere but not one actually shown in the question.. 

I love that one (now).

The hidden question is to name all 5 components...  Such as the inductor, and the antennae, and perhaps the arc lamp (in the switch), and perhaps its oscillation frequency.. (wire resistance being only half a mark)

Reminds me of an old Scottish Power story from Jim Toal about a switch (South Gyle IIRC) that was always burning out from arcing, and when he attended, asked the surprised lineman to connect a spare drum of cable to the switch.

Turned out that the switch was lightly loaded, and when interrupted the resonant frequency was high enough that the arc never had time to quench. Adding the extra capacitance of the unterminated drum of cable drop the frequency enough to stop the arcing. Proper fixes were planned later...

The one which always seemed pointless at the time, but then I actually probably found I used the most in my career, were the ones along the lines of "a box with four terminals, with resistive components inside, has the following voltage and current readings when a voltage of X is applied to these terminals with short circuit / open circuit across these terminals, draw the equivalent circuit". At the time I remember thinking "or just open the box and look inside?" but then spent years using Thevenin,  Kirchhoff and Norton more than any other theorems other than Ohm's law, and realising why we were given that question over and over again...

I even nearly used it last week at home, I'm making some tweaks to a passive pre-amp on my stereo and needed to know what value pots I'd used on the volume and balance controls, and what the equivalent resistances were at the volume we normally listen at, and was remembering that I could work all that out without opening the box. But then cheated and opened the box anyway! (To solve the somewhat more basic problem of the mains switch, which switches all the AV equipment, which had finally burnt out after 30-odd years for the reasons Mike and Philip mention! It's a switch and a chunky relay now.) 

oops - so it was.

Might have to look into the circumstances a bit more, but with 7/029 having a c.s.a. of neatly 3mm² - directly embedded or clipped direct (no thermal insulation in original 1920s houses) - it must be good for at least 30A - so an extra 2/3rds of 10A-ish shouldn't be an issue in practice. If you're relying on the BS 3036 for overload protection and have to take the 0.725 factor into account (or the "course" ratings as they were) it does start to look a bit slimmer (but then is the extra load available to make an overload feasible?)

On the other hand if the three sets had different orientations (e.g. one East, one South and one West) so they'd never be generating full whack at the same time, the whole question might be moot.

Or just say it's not a BS 7671 matter?

What would you say if you found two 3kW appliances plugged into the same double socket?

   - Andy.