As there has not been any activity in here lately...

happysteve · Feb 1, 2014

4km of 2.5mm T&E, huh? Hmm...

Putting aside the insanity of this setup for one moment (voltage drops ahoy), you've got a pretty reasonable stray capacitance between L and E. Let's say it's 150pF/m (IET Forums - Help Intermittent RCD) that's 4k x 150pF = 0.6uF.

i = C (dV/dt)

V= 230 cos(2.pi.50.t)

(let's forget the bit about RMS for now, as we'll just quote RMS current)

So max (dV/dt) is 2.pi.50.230 = 72.3kV/s

so i = 0.6u x 72.3k = 43mA.

Should I also be taking R into account (18mOhm/m = 72 Ohm)?

Silly Sausage · Feb 4, 2014

Hi Steve,

Looks good!

I've just worked it out....43mA as well.

C = 4000 * 150e-12 = 0.6e-6
Xc = 1/(2.pi.50.C) = 5305.xxxx
I=230/Xc = 43mA

AC Voltages and Currents are always r.m.s. values unless otherwise indicated, eg V[SUB]pk[/SUB]
Forget the cable resistance, no loads connected.
It could get very complicated then, taking voltage drop along the cable lengths into account!
Purely academic that would be!!!

I got my 150pF figure from here...9.2 page 38
http://www.google.co.uk/url?sa=t&rc...=k_bAaxJjJ-Gks6HR1BmFcA&bvm=bv.60444564,d.ZG4

It's well worth downloading and reading (if you haven't already).

OK boys, next problem...

happysteve · Feb 7, 2014

Thanks, Archy... That's a great reference! Have only skimmed it, will download it n read it on summat bigger than my mobile at some point. Cheers!

Your way's neater... Always did wonder where the 1/2.pi.f.C thing came from, guess that answers the question!

I will generally bow out of the easier questions on here to give others a go... But that one really threw me, took ages thinkin about it, n even when I'd got summat I wasn't sure. Cheers!

Silly Sausage · Feb 12, 2014

Part 2.

Same installation, same conditions, energised, no loads connected.
Circuit 3, Radial, 200m, CPC disconnected at DB.

What voltage would you read on the CPC, with your other probe on the MET (basically the Neutral point of the source, resistance of conductors is negligible), using...

1) An analogue Voltmeter with an input impedance of 10kOhm
2) A DMM with an II of 3.3MOhm.

(Both of the above are purely resistive)

Then, in 2 or 3 sentences, explain in simple terms, describe why.

Silly Sausage · Feb 21, 2014

A quickie...

3 phase machine, identical windings i.e. balanced, 3 terminals, all connections in good order; basically it's in perfect order!

There is no power, no documentation etc and you can't open it up for inspection.
How would you determine if it is in Star or Delta configuration?

MDJ · Feb 21, 2014

Archy Styrigg said:
A quickie...

3 phase machine, identical windings i.e. balanced, 3 terminals, all connections in good order; basically it's in perfect order!

There is no power, no documentation etc and you can't open it up for inspection.
How would you determine if it is in Star or Delta configuration?

Open the lid on the connections and have a look at they way it is wound lol

Silly Sausage · Feb 21, 2014

MDJ said:
Open the lid on the connections and have a look at they way it is wound lol

Oi, shut it!
Do I look like some blonde $%£^???

happysteve · Feb 21, 2014

Tricksy (for me, anyway...) The resistance measured between two terminals on a delta will be 1/3 that of a star... So if we know summat about the power rating, maybe use that to tell you? This is only a guess, though... Perhaps I'll have to ponder some more. Motors is summat I definitely would like to learn more about...

Silly Sausage · Feb 22, 2014

You need to know nowt about motors...
all you need is an OHMmeter and a piece of wire.
Giving the game away now!

EDIT: Instrument change, sorry chaps!

Sarz11 · Feb 22, 2014

Archy Styrigg said:
Oi, shut it!
Do I look like some blonde $%£^???

Oooh I love it when you talk about me like this... do it again. For the record I prefer blonde **** wit it has a nicer ring to it. Now we both learnt something today didn't we in this post Im glad I read it

(I am actually glad I read it!)

happysteve · Feb 22, 2014

Archy Styrigg said:
You need to know nowt about motors...
all you need is an OHMmeter and a piece of wire.
Giving the game away now!

EDIT: Instrument change, sorry chaps!

See, I've been thinking about this (along those lines) but I don't think I'm quite there yet.

Say the winding resistance is R.

On a delta configuration, you will measure (2R)/3 between any two terminals; on a star you would measure 2R. If you shorted two terminals together (not the terminals you're measuring the resistance across, obviously!), using Archie's bit of wire, then on the delta the resistance would change to R/2, on the star it would change to (3R)/2. Either way, your resistance goes down to 3/4 of its original value:

Delta: from (4R)/6 to (3R)/6 (=3/4)
Star: from (4R)/2 to (3R)/2 (=3/4)

No difference, so this obviously isn't the answer you've got in mind.

More thinking...