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me1

Hi I was give this calculation to do! First have I done it right? And is this the same as calculating parallel circuits?

In a 230V DB 7 different circuit give out an installation resistance test of 200, 200, 150,100,50, 25 and 2.
I came up with

RT= r1+r2+r3+r4+r5+r6+r7


RT = 1/200 + 1/200 + 1/150 + 1/100 +1/50 + 1/25 + 1/2

RT = 0.005 + 0.005 + 0.007 + 0.01 + 0.02 + 0.04 + 0.5 = 0.59

RT = 1/ 0.59 = 1.69 Momhs

I hope that I have explained this correctly?
 
looks good to me. as a quick check, the answer must always be < the lowest parallel resistance.
 
Your first formula (RT= r1+r2+r3+r4+r5+r6+r7) is for series resistances but you've worked it out correctly.
 
^^ Agree with Marvo^^ - you're first formula is for resistances in series, NOT parallel. Daz
 
You’re a bit out on you’re final addition or the invese, I get 1.73MΏ but as Tele and Marvo will tell you I cheat. They have the spread sheet I use.
 
You’re a bit out on you’re final addition or the invese, I get 1.73MΏ but as Tele and Marvo will tell you I cheat. They have the spread sheet I use.
I think the discrepancy is because you rounded 1/150 to being 0.007 when it's actually 0.0066666. Rounding up and down in the middle of a calculation can make a large difference to the accuracy of your final answer. On these types of calcs I'd be working to at least 6 decimal places but if your tutors have told you differently then I guess you should follow their advice.
 
As I said I cheat, I wrote a spreadsheet for calculations. Sorry but I won’t give it to trainees to cheat with.

For Excel
=1/((1/200)+(1/200)+(1/150)+(1/100)+(1/50)+(1/25)+(1/2))

And I’ve just realised I’ve gone wrong, I missed out the 100MΏ so it now comes in at 1.704MΏ.
It depends on the degree of accuracy, I’m working to 6 decimal places. Not that that helps. No meter is going to be that accurate in the MΏ range.
 
Hi I think the formula for working out resistance in parallel is 1/RT= 1/r1 + 1/r2 + 1/r3 etc I might be slightly wrong as a little rusty on the formula so might be worth double checking
 
I've been watching this thread develop.....

Whilst his workings are correct (with roundings) for parallel resistances; his initial formula "RT= r1+r2+r3+r4+r5+r6+r7" is for series. We are all agreed on that.

Now, because we've stated it's a series formula, me1 thinks we were referring to his formula AND his workings.

So, he's now confused and is thinking "....hmmmm, well if that's a series calculation, best I ask how to do a parallel one...."

I don't think he was asking for a 'cheat' (i.e. spreadsheet). I think we have genuinely confused the guy!

Hope i'm right or i've just wasted another ten minutes of my life!
 
AD I tend to do all calculations in excel. I’ve used spreadsheets since the days of Lotus Symphony (many moons ago). On paper before that.
The calculation for parallel above is in excel format, it’s how I work.
 
......Now, because we've stated it's a series formula, me1 thinks we were referring to his formula AND his workings.

So, he's now confused and is thinking "....hmmmm, well if that's a series calculation, best I ask how to do a parallel one...."

You might be right Arsey I think he's confused and I know I'm certainly confused so to clarify;

The insulation resistance readings of 7 circuits on a DB would effectively be in parallel so you'd resolve those individual resistances into one overall resistance using the formula 1/RT= 1/r1 + 1/r2 + 1/r3 + 1/r4 + 1/r5 + 1/r6 + 1/r7.

This is exactly what you did in your calculation RT = 1/200 + 1/200 + 1/150 + 1/100 +1/50 + 1/25 + 1/2 and your answer was basically right except maybe for a little bit of over-zealous rounding of the figures.

Series resistances are resolved into one overall resistance by using the formula RT= r1+r2+r3+r4+r5+r6+r7. This was the first formula you gave us in your opening post but you quite rightly didn't use it in this particular calculation, I assume you just put it there to keep us on our toes ;).

Just as an aside, we have a trainee sub forum that I suspect you may qualify for access. Please have a read of this and do the necessary, if you're presently on a training course then one of the Admin will give you access.
 
AD I tend to do all calculations in excel. I’ve used spreadsheets since the days of Lotus Symphony (many moons ago). On paper before that.
The calculation for parallel above is in excel format, it’s how I work.

Tony, just to be clear, my "cheat" reference was meant in the context of spreadsheets for trainees. Maybe i've misinterpreted the tone of your message but I feel chastised, undeservedly so!!

Oh......and you omitted slate & chalk.

Edit: Meant to include a smiley to show this is posted in good humour!
 
I've been watching this thread develop.....

Whilst his workings are correct (with roundings) for parallel resistances; his initial formula "RT= r1+r2+r3+r4+r5+r6+r7" is for series. We are all agreed on that.

Now, because we've stated it's a series formula, me1 thinks we were referring to his formula AND his workings.

So, he's now confused and is thinking "....hmmmm, well if that's a series calculation, best I ask how to do a parallel one...."

I don't think he was asking for a 'cheat' (i.e. spreadsheet). I think we have genuinely confused the guy!

Hope i'm right or i've just wasted another ten minutes of my life!

Hi ArseyDee

Yes you are correct!

So how are parallels worked out please?
 
Or if no calculator, you can find the lowest common denominator of your resistor values

Divide each resistor value into the common denominator and add them all together

Then divide your common denominator by the total you got from the step above

Theres your answer

Cheers

Matt
 
Or if no calculator, you can find the lowest common denominator of your resistor values

Divide each resistor value into the common denominator and add them all together

Then divide your common denominator by the total you got from the step above

Theres your answer

Cheers

Matt

I suspect that all the younger guys would still need a calculator to do this!! lol!!

Don't think schools these days do mental arithmetic as part of the learning curriculum... Hell, many leaving school these days can't do math calculations in written form!! lol!!
 

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