Look up the maximum Zs values for MCB's, Fuses, MCCB's and ACB's with 100% values and the temperature adjusted values all in one place.

Calculate and look up the maximum permitted Zs value ( earth loop impedance ) to BS 7671. Max Zs Values also includes the 80% temperature adjusted values and a some MCCB 60947-2 manufacture Zs charts.

- 100% BS 7671 Zs Values
- 80% On Site Guide Zs Values
- Printable 60947-2 MCCB Zs Tables

- 0.4 (s) second disconnection times
- 0.2 (s) second disconnection times
- 5 (s) second disconnection times

> Common BS EN Devices:
Scroll through the list of protective devices and select the device rating to calculate the maximum permitted Zs value for your selected protective device and rating combination.

> 60947-2 MCCB's Zs Tables:
Look up the maximum permitted Zs tables for a selection of MCCB manufacture devices (some).

Maximum Zs Features:
+ Maximum Disconnection Times
+ BS 7671 18th Edition Zs Tables
+ Fuse's
+ MCB's
+ RCBO's
+ RCD's in TT earthing systems
+ Max REC Ze values
+ Ze + R1+R2 = Zs calculator
+ 100% values (BBB)
+ 80% values (OSG)
+ 0.4 second disconnection times
+ 0.2 second disconnection times
+ 5 second disconnection times
+ Uo 55v (U 110v) single phase and Uo 63.5v (three
phase) Zs values
+ BS EN 60947-2 MCCB manufacture device charts

A maximum Zs calculator for iPhone and iPad.

Check out our collection of electrical apps such as the "Pro Certs" Electrical Testing & Certification App or visit for more info.

What’s New

Version 3.1.7

Added manufacture BS 3871 Type 4 max Zs values.

Ratings and Reviews

4.5 out of 5
16 Ratings

16 Ratings

Astect Gibbo ,


I downloaded this app last year and it was great very handy, There was an issue with the software but the developers was extremely helpful and went out of their way to fix this issue, I’ve recently downloaded the updated version and can honestly say it’s gold excellent stuff

Developer Response ,

Thank you for your feedback.

Ryan 'hamster' Hamdy ,

Handy pocket guide

Great app!!! I have been using this app for a few years now and I can not fault it, it’s easy and convenient to use and rather than having to overthink and take you away from the everyday job it saves time.

Developer Response ,

Thank you for your feedback. You may also like our fully loaded electricians app “Electrical tools and reference” for iOS.

Cjiphone4s ,

Great app but...

Love this app very handy when needing to remember figures,values etc. However since iOS 14 sadly does no longer display correctly on my iPhone xs which deems some of the functions unusable atm - hopefully a fix for this is in the pipeline?.

Developer Response ,

Hi, can you please send us a screenshot to of the issue so we can look into a fix? UPDATE 12/12/2020: Thank you for the screenshot, we believe we have fixed the issue and the new update is now live.

App Privacy

The developer, Pro Certs Software Ltd, indicated that the app’s privacy practices may include handling of data as described below. For more information, see the developer’s privacy policy.

Data Not Collected

The developer does not collect any data from this app.

Privacy practices may vary based on, for example, the features you use or your age. Learn More


  • Family Sharing

    Up to six family members will be able to use this app with Family Sharing enabled.

More By This Developer

You Might Also Like