What are Zs Amps & Volts?

Zs and Ze

There are different units in electrical installations and every one of them play a key part in our modern everyday lives. On the surface these units can look basic, but what do they truly stand for?

Zs represent earth fault loop impedance and consists of Ze and (R1+R2). R1 is the phase and R2 is the CPC. These are measurements of the fault current from the supply transformer to the building. It covers the entire electrical cycle. If Zs are too high, not enough fault current can flow and the MCB may not trip at all. On circuits after 17th edition, RCDs will be more common and will trip before MCBs.


Simply put, volts and amperes (amps) are measurements of electricity.

Volts are the unit of electrical potential difference or the size of the force that sends the electrons through a circuit. There are 2 different voltage types, DC and AC. DC means direct current, an unidirectional flow of electrical charge. This current may flow through a conductor like a wire but can also flow through semiconductors and insulators. AC means alternating current which periodically reverses direction, contrasting direct currents only flowing in one.


Checking amperage helps determine if something is pulling more energy than necessary. An ampere is a measurement of the rate of electron flow or the current in a conductor. An easier way to think of it is one amp represents one coulomb of electric charge.


Fault Loop Impedance: Successfully tripping under fault conditions.

Ze: External fault loop impedance at mains, usually assessed based on your enquiry.

Zs: Internal fault loop impedance at DB.

CPC: Circuit Protector Conductor (or Earth).

RCD: Residual Current Device, designed to make and break currents under normal working conditions.