Calculate Earth Loop Impedance - Cable >35mm2 Impedance Data

[Chr!s]

Hi all,

Where do I get phase (Z1) and earth (Z2) conductor impedances to calculate earth fault loop impedance?

I have seen (R1 + R2) tables which give combinations for cable sizes up to 35 mm2. What about cables > 35 mm2?

Can I use voltage drop tables (for phase conductors) from Appendix 4 in BS 7671? These are impedance of the entire circuit though aren't though (in the case of single phase ccts includes phase and neutral conductors - so divide by 2?)?

Thats perfectly fine, divide by 1.73 for three phase assuming balance loads.

I want to calculate earth loop impedance for any size phase and earth conductors. Where do I get the impedance data from the Standards?

Apendix four or manufactures data

Thanks,

Battler.

Regards Chris

 

[malcolmsanford]

150mm cable R1 R/m = 0.124 ohm

75mm CPC R2 R/m = 0.268 ohm = 0.392 ohm

For a 60m metre length will be 0.392 X 60/ 1000 = 0.02 ohm @ 20c

What don't you understand .............is that too complicated for you.

 

[BATTLER.]

75mm CPC R2 R/m = 0.268 ohm = 0.392 ohm

Well actually, mathematically this makes no sense. I assume you mean:

R1 + R2 = 0.124 + 0.268 = 0.392 Ohms/km

For what sort of cable is this data for? What type of installation? Where did the data come from?

For cables > 35mm2 reactance of cables needs to be considered and this is affected by the installation method. How can we use voltage drop values directly from BS7671?

Here's a scenario:

Single-core 90degC armoured Cu single phase cable of 240mm2 installed as Method C (Touching) from TABLE 4E3A. CPC of 70mm2. What's the earth loop impedance?

 

[malcolmsanford]

Can you tell me what method C as got to do with an EFLI ? nothing is that answer

From the link I posted to you if you bothered to read it

240mm r/m is 0.0754 ohm

70mm r/m is 0.268 ohm

So combined it will be 0.3434 ohm , so now all you need is the length of the cable and hey presto us thick stupid electricians have worked it out for you.

Also you will need the external earth fault loop impedance to complete your calculation.

 

[Engineer54]

Do you recall this statement of yours malcolm? Do you have a problem?

I apologise if my questions are too technical for this electricians forum. Continue being ignorant and thinking that you know how to do everything when you clearly do have no idea.

I thought I made it very easy; read the title of the thread:



Again, you're only electricians and I apologise, I shouldn't have expected that much.

You arrogant git.... If these guy's are ''Only'' electricians , and you consider yourself something better, ....Why are you asking, what IS basically simple stuff, that a man of your perceived self esteemed caliber should know ??

So just remember my friend, this electrician knows the answer ....YOU Don't!!!! Self perceived arrogant gits like you, are two a penny in this game, a ''Legend in their own minds''.....

 

[BATTLER.]

malcolm, there is a reason why they have separate voltage drop tables for each type of cable and installation conditions. That is because cable impedances are dependent on these factors. Installation method has everything to do with cable impedances, especially phase conductors > 35mm2.

(a) I don't agree with using random internet sources for design data.
(b) I don't believe the data you're proposing is applicable or accurate enough for the scenario I have given nor is it applicable for every cable type and installation condition, as you're suggesting.

What is the proper method and what are the data sources we're to use suggested in the Standard? This is not clear to me.

 

[malcolmsanford]

The OSG will give you cable resistance up to 50mm on page 166 Table 9A anything above that you normally would get this from the cable manufacturer. Is this for design purposes ?

THEN DO AS I SUGGESTED IN MY POST 3, if you don't trust the table I linked 2, and why not I don't know, then your only other course of action is to go to the cable maker and get the r/m of the cable from them.

 

[IQ Electrical]

Well actually, mathematically this makes no sense. I assume you mean:

R1 + R2 = 0.124 + 0.268 = 0.392 Ohms/km

For what sort of cable is this data for? What type of installation? Where did the data come from?

For cables > 35mm2 reactance of cables needs to be considered and this is affected by the installation method. How can we use voltage drop values directly from BS7671?

Here's a scenario:

Single-core 90degC armoured Cu single phase cable of 240mm2 installed as Method C (Touching) from TABLE 4E3A. CPC of 70mm2. What's the earth loop impedance?

Hi, I'm another thick spark, try this:

The volt drop tables of appendix 4 of BS7671:2008 can be used as a source of the resistance AND reactance per metre of the cables tabulated.

Correcting from maximum operating temperature to 20 Degrees for the resistance element (mV/A/m)r that is dividing by 1.2 and dividing both the resistance (mV/A/m)r and inductance (mV/A/m)x by 2 as the tabulated voltage drop includes voltage drop in the line and neutral.

 

[SW1970]

Some of these new threads by those-of-few-posts have got to be wind ups.

 

[Chr!s]

Well actually, mathematically this makes no sense. I assume you mean:

R1 + R2 = 0.124 + 0.268 = 0.392 Ohms/km

For what sort of cable is this data for? What type of installation? Where did the data come from?

For cables > 35mm2 reactance of cables needs to be considered and this is affected by the installation method. How can we use voltage drop values directly from BS7671?

Here's a scenario:

Single-core 90degC armoured Cu single phase cable of 240mm2 installed as Method C (Touching) from TABLE 4E3A. CPC of 70mm2. What's the earth loop impedance?

I assume you know how to calculate the resistance.

Providing the cables are touching then i would use the the average reactance of the two conductors, if they are not close then you will need to calculate the reactance.

Also as you are using a reduced csa for the protective conductor, it may be prudent to use the average of the working and final cable temps.

Regards Chris

 

[netblindpaul]

Batler,
if you are going to go this far into it you need to know the characteristics of your load at the end of the cable.
The pf, and the full monty really.
Once you have this then you are in a full position to assess the full impedance of the cable in relation to your fixed load.
if your load is not fixed then you are wasting your time.

 

[Guest123]

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