A Little Help With Figure's Please

[Ohm]

Examples of insulation on individual circuits

The following I.R results were measured on individual circuits of a domestic installation,

Cooker - 200M hms

Sockets - 200M hms

Lights - 200M hms

Shower - 200M hms

Outside Lights - 1.2M hms

Line to Earth

What would the reading be if the test had been performed on the whole installation as apposed

to individual circuits?

Do the test results comply with regulations ? What action should be taken ?

Give possible reasons for the readings on the outside lights circuit, if the outside light circuit were 

protected by a 30mA RCD, would it trip ?

Answer - The results of the 5 resistances in parallel is 1.17M hms  .( This I can work out using decimal method)

All individual circuits are in excess of 1M hms   so they do they comply with regulations, it should be noted on certificate that there is a latent fault, which requires further investigation. 

A possible reason for this low reading is water ingress in the outside light fittings, By using ohm's

law, the earth fault current would be 0.19mA, so a 30mA RCD would not trip.

Question - How low would the Phase to Earth I.R reading have to be to trip the 30mA RCD ?

Answer - Using Ohm's law, the value is 7666.7 hms

Now the bit I cannot work out is how they have arrived at earth fault current of 0.19mA ?

I have tried all combinations that I know of and cannot get an answer of 0.19mA, can anyone

give me some guidance on this please,                                                                                               

also the value 7666.7 hms   I can nearly get the correct 

value but decimal point is in the wrong place - 766.66, 230v / 0.30 were am I going wrong?

This is home study coarse work 2394/5, Thanks for any help.

 

[Zeespark]

Phase to I.R.?

Andy Guinness

 

[Evans Electric]

Are you meggering through PIR,s  or photo cells ?

 

[Ohm]

Hi Andy,

Phase to Earth

Just basic circuits

 

[steptoe]

Are you meggering through PIR,s  or photo cells ?

its a homework question Deke,

should be in the student section really,  :|

 

[ProDave]

The readings of 200M  hms  are probably not real.

Many testers won't read above 199M hms  so in reality the true resistance of the good circuits may be infinity, so the reading of the whole lot in parallel could well be just the 1.2M hms  of the suspect circuit.

The fault current is V / R  So 230 / 1,200,000 = 0.19mA

For 30mA it's R = V / I  so 230 / 0.03  = 7666.66666 hms

 

[Marvo]

Give possible reasons for the readings on the outside lights circuit........

A possible reason for this low reading is water ingress in the outside light fittings,

If there's electronic transformers or ballasts (or even CFL lamps)they can present small amounts of leakage through surge arrestors or noise/RFI suppression circuits.

........230v / 0.30 were am I going wrong?

As per Dave's calculation above, 0.30A you've used is 300mA not 30mA. Also depending on the number of decimal places you're expected to work to you can round up the 7666.666 reoccurring to 7666.7

 

[Ohm]

Thanks ProDave and Marvo for putting right,  sometimes its the most simplest of things but you just cant see it.

Guinness   Guinness  on me

 

[apprentice87]

Got to admit, this is one of the best posts i have read.... How much better is it that someone has done all their workings out and posted them rather than just asking "what is the answer"

Well done that man!!!

john...

 

[Manator]

Got to admit, this is one of the best posts i have read.... How much better is it that someone has done all their workings out and posted them rather than just asking "what is the answer"

Well done that man!!!

john...

That is probably why he got such good answers and is now clear on where and how he went wrong.

Indeed well done that man, and thank you to all who responded with some very good answers.

 

[Andy™]

Got to admit, this is one of the best posts i have read.... How much better is it that someone has done all their workings out and posted them rather than just asking "what is the answer"

Well done that man!!!

john...

+1. he done his best before asking, unlike many who just copy all the questions and expect all the answers

 

[Blue Duck]

A forum with a difference.... hard work pays off here!  slackers will be sent back to diynot or screwix forums & have everything spoon fed to them...

 

[ParkySparky]

+1 excellent post and thread.

This is what the forum is about, good, honest non Mickey taking answers.

As said, the way the question was asked and then showing your own knowledge and workings get you the respect of the others to get the answers.

The good part is that the correct answer was just not posted but a explanation as to how to get there and also a reason as to why you got to your answer.

I doff my hat gents