Earth fault loop impedance

[Anonymous user]

1. Once we got Zs from calculation, So the Zs value referred between distance of cable of Phase & Ground (PE). kindly advise & explain more on how to select the cable size of Phase & Ground (PE), because the cable size is not same.

2. the Zs value calculated came from Transformer or sources point to earh Fault, What if, There are more level of that points.

how we can calculate the Zs? For example :

200kVA/415/230V, Z= 6% (consists MDB, EMDB, EDB and Final load (Load center))

3. HOw can we consider the cable size of Ground (PE) ? If our trip unit of CB has Earth Fault Protection or feeder has Earth Fault Protection Relay

So I enclosed the Zs details of CB from SE website

Kindly compare & explain about how different between points & purpose of IEC 60364 & BS 7671 

Access the "Moulded Case Circuit Breaker Zs Values" PDF

[MathieuGuillot · Accepted solution]

Hello

Thanks for this question at the heart of protection against electric shock in TN system.

First comment : there is no major difference between IEC 60364 part 41 and part 54 and BS7671 Chapter 41 and 54. The principle are the same. BS allows to use extraneous conductive part or cable tray as PE where IEC does not but requirement for impedance are the same as for a dedicated conductor.

Regarding the Fault loop impedance calculation you may find some helpful material in Electrical Installation guide : https://www.electrical-installation.org/enwiki/TN_system_-_Principle 

The fault loop impedance is the sum of source impedance, line impedance and PE impedance. if the cross section of PE is lower than the line it will be considered in PE impedance. The source impedance can be calculated from transformer caracteristics. In your example : 415^2 / 200 000 * 0,06 if Resistive part is neglected.

The accurate calculation including Resistive and Inductive part of each part of the loop usually requires software tools. Some simplified methods exist (see electrical installation guide).

I will ask Neville Watkins to comment on the table we provide for the selection of circuit-breaker according to the Zs.

Hope it helps.

Happy new year

[Anonymous user]

Regarding loop impedance, I have a query. How is IEC 60909-0 and IEC TR 60909-4 addressing the issue of 3Zn? When we have a L-G fault current return path, it will flow back in PE conductor accompanying the phase conductors. 

Effectively, the L-G fault current = 3 x U0 / (Z1+Z2+Z0+3ZN). I do not see 3Zn in the calcs as presented by IEC TR 60909-4. In scope sec 1 of the IEC 60909-0, it is written that L-G is beyond the scope of this part of IEC 60909 (page 7). The mention of fourth line and/or earth in the definition of zero sequence short circuit impedance (page 12) is purely for mutual coupling and not for 3Zn calcs. 

Please give your opinion on this.