Calculating earth loop impedance

[EddieB]

Hi all R1 + R2 +Ze this equates to Zs but does this also equal the earth loop impedance, can I calculate ELI instead of measuring it, or are there other factors involved?

 

[Deleted member 9648]

Zs is the symbol for ELI....you can calculate or measure,although the measured reading may be different to the calculated due to parallel paths.Most prefer to measure but some scheme providers advise that where access to live parts is involved...ie luminaires...you should calculate due to H&S considerations.

 

[nickblake]

Zs is your earth fault loop impedance , you take your Ze at the incomming of the supply normally across the main switch , then on each circuit for example a lighting circuit you take your R1&R2 L and CPC joint together at the consumer unit then you add the reading of the Ze and add that to the reading of your R1&R2 and you'll get your Zs but taking the Ze readings you must disconnect any bonding to water gas etc

 

[i=p/u]

mr puller got it 1, cant add

 

[i=p/u]

what out comes could you have by using the suppliers set values for ze, shorter circuits runs???/if designing??

 

[IQ Electrical]

what out comes could you have by using the suppliers set values for ze, shorter circuits runs???/if designing??

The suppliers values are all worst case scenario's from ESQCR 2002 for design purposes so they actually limit circuit lengths etc.

 

[i=p/u]

so on pages 48-9-50 etc of the on site guide are these circuit lengths done on worst case scenario????

 

[brizospark]

it it also acceptable to apply this inversely and use zs - ze to calculate r1+r2?

 

[IQ Electrical]

so on pages 48-9-50 etc of the on site guide are these circuit lengths done on worst case scenario????

Yes, if you look at the Zs values at the right hand side, you'll see the 0.8 and 0.35 Ohm values.

 

[IQ Electrical]

it it also acceptable to apply this inversely and use zs - ze to calculate r1+r2?

No, the result does not 'back calculate' because of parallel paths, you end up with vastly reduced R1+R2 values in most cases.

 

[nickblake]

it it also acceptable to apply this inversely and use zs - ze to calculate r1+r2?

the problem with back calculating as i call it is parrallel earth paths if you have these paths when you carry out your Zs then the R1&R2 value would be incorrect

 

[Chr!s]

The suppliers values are all worst case scenario's from ESQCR 2002 for design purposes so they actually limit circuit lengths etc.

Dont think you will find any values in ESQCR, best place for max Ze is in dnos earthing manual.

Many DNOs allow upto .8 on a tncs, though this will restrict the size of supply.

Calculating Ze is only really possible on a private supply, there you would need to know all the various impedance of th TX etc. On a public supply the dnos have computerized models which will give you a approx value. Last option would be to measure.

 

[brizospark]

What about for the purpose of carrying out a PIR / EICR?

In large factories etc. it is sometimes inappropriate to go into d.b.s and connect L+E together etc. to find out R1+R2

Surely the most important thing is making sure the Zs of a circuit is OK. Is the R1+R2 really relevant on a PIR?

 

[telectrix]

IMO, R1 and R2 are relevant on an initial verification as a measure of circuit integrity before energising your new circuits. on a PIR, the circuits are already energised and in use, so measuring Zs and obtaining a satisfactory reading is sufficient to assume a satisfactory R1 + R2.

 

[i=p/u]

just original post asked about calculating it, but your right as long as zs is ok