Maximum Zs Values


Max Zs Values

18th Edition Max Zs values.

✔ BS 7671 100% Values
✔ On Site Guide 80% Values
✔ 0.2s Disconnection Times
✔ 0.4s Disconnection Times
✔ 5s Disconnection Times
Extra Zs Calculators and Tables

Max Zs Values App for iOS & Android

BS EN 60947-2 MCCB Zs Tables Includes some manufacturer BS EN 60947-2 MCCB Zs value tables
55v and 63.5v Zs Values Includes a calculator which calculates Uo 55v (U110v) single phase and 63.5v three phase Zs values from table 41.6
RCD's in TT Systems Zs Values Includes a max Zs calculator for RCD’s in TT earthing systems from table 41.5
Maximum Disconnections Times Disconnection times from table 41.1 with a bit extra info


  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot
  • ‎Max Zs Values Screenshot


Max Zs Values is an easy to use Zs calculator for looking up and calculating the maximum permitted Zs value for RCBO’s, RCD’s, MCB’s, fuses and some manufacture MCCB’s. Earth loop impedance (Zs) is measured in ohms.

Spending hours hunting for max Zs tables and Zs values is now thing of the past with this Max Zs Values app.

Max Zs Tables

  • Table 41.1 – Maximum Disconnection Times
  • Table 41.2 – Maximum Zs Values for Fuses at 0.4s
  • Table 41.3 – BS EN 60898 and 61009-1 Zs Values
  • Table 41.4 – Maximum Zs Values for Fuses at 5s
  • Table 41.5 – Zs Values for 61009-1 & 61008-1 in TT Systems
  • Table 41.6 – Zs Values for Uo 55v (single phase) and 63.5v (three phase)
  • DNO (DSO) Ze Values & Earthing System Types Diagram

Disconnection Times

  • 0.2 seconds (s)
  • 0.4 seconds (s)
  • 5 seconds (s)

Max Zs Calculators

  • Common BS EN Devices
  • Manufacture 60947-2 MCCB Tables
  • MCCB Explanation of Settings Symbols
  • RCD’s in TT Earthing Systems
  • REC Ze Values
  • Uo 55v and 63.5v Max Zs Calculator
  • Zs – Ze = R1+R2 Calculator

Max Zs Charts

  • BS7671 18th Edition Zs Values (100% Values)
  • BS7671 On Site Guide Zs Values (80% Values)
  • Maximum Disconnection Times Table for TN & TT Earthing Systems


  • Print the 60947-2 MCCB Zs Tables
  • Request New Manufacture Zs Tables
  • Fully Supported
  • Well Maintained


  • Easy to Use

    Easy to use UI navigation menus
  • Multiple Zs Calculators

    Includes many max Zs calculators
  • Extensive Zs Values Database

    Fully equipped Zs database
  • Works Offline

    Works offline, even in airplane mode!
  • Print

    Print the MCCB Zs tables
  • Fully Supported

    Our dedicated support team are here to help, contact support


Date: August 2021
- Added BS 3871 Type 4 max Zs values (various manufacturers)
- Added a news feed from our blog

Date: February 2021
- Some layout improvements
- Corrected a spelling typo

Date: September 2020
- Added: MCCB Explanation of Setting Symbols
- Added: DNO Ze Values & Earthing System Types Diagram
- Changed: Re-worded reference from REC to DNO / DSO

Date: May 2020
- Improved UI
- Added a custom document viewer to view the MCCB Zs tables

Date: September 2019
- Added the ability to print the MCCB Zs tables
- Added more BS EN 60947-2 MCCB manufacture Zs tables
- Reduced app download size

Date: 2019
- Complete new redesign
- Complete new UI
- Back-end improvements
- Added a few new calculators

Date: 2018
- BS 7671 18th edition update
- Zs values updated

Date: 2015
- BS 7671 17th edition amd 1 update
- Zs values updated

Date: 2013
- Maintenance update

Date: 2013
- iOS 6 update

Date: 2013
- Fixed BS88 bug

Date: 2012
- New protective devices added
- Zs database updated

Date: 2012
- Sharing function improved

Date: 2012
- 1st release
18th Edition Max Zs Values

Max Zs Values

BS7671 18th Edition Zs Values

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FAQ's & How To

How to calculate max Zs

In order to calculate the maximum permissible Zs value we need to know the following data:

  • la = The amount of current required to trip the MCB within the given time.
  • In = The rating of the protective device in amps.
  • Uo = Voltage to earth

Calculate Zs Formula:

Uo x 0.95 (Cmin) / la = Max Zs (100% value) x 0.8 = Max Zs (80% value)

Example: BS EN 60898 Type B 32 Amp

Ia: 5×32(A) = 160(A)

Uo: 230v x 0.95 = 218.5v

218.5(Uo) / 160 (la) = 1.36525, 100% Zs value rounded to 1.37 Ω.

1.37 (100% Zs Value)  x 0.8 = 1.096, 80% Zs value rounded to 1.1 Ω.

Which Zs values should I use, 100% or 80% values?

It is generally recommended that the 100% max Zs values should be recorded on an electrical installation certificate or condition report.

However the actual measured Zs values obtained should be compared to the 80% temperature adjust values, this is to allow for the rise in cable temperature in fault conditions.

Should a measured Zs value exceed the 80% temperature adjusted Zs value then this would be considered a C2 observation.

  • 100% Max Zs values = Record on paperwork.
  • 80% Max Zs values = Compare to actual measured Zs values.

Can I request a new feature?

Yes, we welcome feature requests and suggestions, you can send us your feature request here.

How to calculate Zs

To calculate Zs for an existing circuit is a simple calculation using the known Ze and R1+R2 values, however we have added a Zs calculator anyway for convenience.

How to calculate R1+R2

To calculate R1+R2 is a simple calculation using the known Ze and Zs or Zdb values, however we have added an R1+R2 calculator to the app anyway as it can be very handy if you have multiple R1+R2 calculations to make.

How to contact our support team

You can contact our support team from the app or via our web contact form, contact support.


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An extensive database of maximum Zs values.

✔ Calculate Max Zs
✔ Calculate Zs
✔ Calculate R1+R2
✔ 100% and 80% Zs Values

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