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Topic Title: Insulation Resistance
Topic Summary: one for the theorists
Created On: 20 February 2014 11:16 AM
Status: Post and Reply
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 20 February 2014 11:16 AM
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marclambert

Posts: 304
Joined: 23 June 2010

My line of thinking is this....
If IR reduces with length (in theory at least)
And with additional circuits,
It's just parallel resistances after all.

According to City & Guilds (in one of their exam success books) a change in csa has no effect on IR.
I thinks this is wrong as there is a greater surface area for the voltage to "leak across" as there is with an increase in length, so the IR will decrease. I accept that this is probably immeasurable in the real world. But do you agree with the theory?
Thanks in advance.
Marc
 20 February 2014 11:47 AM
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SKElectrical

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Joined: 01 February 2009

Sounds right
 20 February 2014 11:56 AM
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geoffsd

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Joined: 15 June 2010

It depends what you mean by 'decrease'.
 20 February 2014 12:09 PM
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marclambert

Posts: 304
Joined: 23 June 2010

decrease... as in go down?
So a circuit has an IR of 50Mohm, double it's length and (theoretically) it decreases.
Add additional circuits to an installation and again the IR reduces. Conversely remove circuits or decrease lengths and IR increases.
Change the csa...and what happens?
is that clearer Geoff? Sorry if my OP was not clear.
regards
Marc
 20 February 2014 12:24 PM
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rocknroll

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The clue is in the word 'insulation' resistance, the csa of the copper or length is a bit of a red herring, the thickness of the insulation and length of the insulation is where you need to search. A 1mm cable insulation thickness is probably the same thickness as a 25mm cable so the csa of the copper is not relevant.

Insulation resistance:

The ratio of the DC voltage applied to the terminals of a capacitor and the resultant leakage current flowing through the dielectric and over its surface after the initial charging current has ceased expressed in megohms or as time constant megohm x microfarads.

Leakage current:

Measure of the stray direct current flowing through a capacitor after DC voltage is impressed on it.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 20 February 2014 at 12:32 PM by rocknroll
 20 February 2014 12:39 PM
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marclambert

Posts: 304
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Yes RnR I appreciate your point. The insulation length is however directly connected to the cable or conductor length?
If the insulation thickness increased with conductor csa then I guess csa wouldn't make any difference. But if it stayed the same then I think my OP still stands? Due to increased surface area.
regards

Marc

and Red Herring or not, that's how it's worded in exam questions
 20 February 2014 12:45 PM
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OMS

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Change the CSA of what ?

IR will be directly proportional to the cross sectional area of the insulation medium - ie thicker insulation, higher IR

The insulation thickness does not vary proportionately with conductor CSA however

It's usual to calculate with a constant(K) for the insulation material and the log mean values for Cable D and d ( D= outside diameter of conductor insulation and d = outside diameter of conductor). Typically in the form of:

R = K x Log 10 (D/d)


So the answer to the question is maybe

Typically, a 2.5mm2 conductor is offered approx 0.8mm XLPE insulation thickness - a 25mm2 conductor would have 1.2mm XLPE insulation thickness

So increasing conductor CSA would tend towards increasing thickness of insulation so would show a consequent increase in IR

I'd be pretty certain that's not what C&G want however

Regards

OMS

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 20 February 2014 03:48 PM
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allaway

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I assume you mean that as you increase the CSA of the conductor, then in theory, yes, the overall insulation resistance will be lower - assuming the insulation is the same thickness and OMS has covered it.

The insulation thickness changes depending on CSA. I did some measurements on a single conductor from T&E from the same manufacturer:

CSA Insulation Thickness
1.5 0.72 mm
2.5 0.78 mm
6.0 0.94 mm
10 1.12 mm


I also took the data sheet for 6491X - Conduit and from that information calculated the nominal thickness - a few assumption such as perfect packing of the 7 strands and the insulation is evenly distributed (there are in fact 6 small ridges)

CSA OD Insulation Thickness (calc)
1.5 3.0 0.81
2.5 3.6 0.91
4.0 4.2 0.97
6.0 4.7 0.97
10 6.1 1.27

However, one other though which comes to mind (from many years back) is that the insulation material MAY actually be thicker than that required to provide the required level electrical insulation so as to give a good level of mechanical protection.
 20 February 2014 04:16 PM
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Zuiko

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Originally posted by: allaway

The insulation thickness changes depending on CSA. I did some measurements on a single conductor from T&E from the same manufacturer:


If you look back at what OMS wrote previously, he said that the insualtion is due to the cross-sectional area of the insulation material. So what you need to do is work this out.

Rather than compare the cross sectional area of the conductor with the thickness of the insulator, compare the cross sectional area of both. Obviously, the thickness and area of the insulation are directly proportional and related to the area of the conductor.
 20 February 2014 06:01 PM
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geoffsd

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Originally posted by: marclambert
decrease... as in go down?

Precisely.

So a circuit has an IR of 50Mohm, double it's length and (theoretically) it decreases.

I may be playing with words but that was my point.
Nothing has 'decreased' as in changed . You are measuring something different.

Add additional circuits to an installation and again the IR reduces. Conversely remove circuits or decrease lengths and IR increases.

The overall value changes but nothing in the installation changes or 'decreases'.

Like the resistance of a conductor increases with length but the resistivity of the copper does not change.
 20 February 2014 06:12 PM
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daveparry1

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I'm sure some of you lot have tooooo much time on your hands, all this "hair-splitting" stuff!

Just joking,

Dave.
 20 February 2014 07:19 PM
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Zuiko

Posts: 518
Joined: 14 September 2010

Originally posted by: geoffsd
The overall value changes but nothing in the installation changes or 'decreases'.



Like the resistance of a conductor increases with length but the resistivity of the copper does not change.


That's not a useful way of looking at it.

It is quite correct to say that the value of the circuit's insualtion resistance increases or decreases, because resistance is a funciton of length. Resistance is a magnitude that can increase or decrease, and does, all the time with temperature, for example.

Another example, is if you doubled the length of your circuit and so doubled the circuit's resistance, you would double the volt drop. So you would have to reduce the resistance.



Resistivity is a material property. Quite a different kettle of fish (and it too can change)
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