Indeed as Mike mentioned 10ms gives a very large number (for 3kA and 10ms -->> energy let through is 90000). The 100% values should be recorded as the maximum permitted Zs value on the electrical test certificate and the temperature adjusted 80% values are used to compare against the actual readings obtained when testing the circuit. 60947-2 Max Zs Values OC or 40 OC,the purpose of the latter value being to avoid the necessity of de-iating thermally sensitive circuit- This is developed in Note 2 to regulation 536.4.203, which states that ‘ If an assembly deviates from its original manufacturer’s instructions, or includes components not included in the original verification, the person introducing the deviation becomes the original manufacturer with the corresponding obligations’. The last bit is to ask ‘how do I calculate the voltage difference between two phases with some other phase angle?’ Clearly this is some function of the amplitude of the phases, and also a function of the phase angle between them. Simplify the question by stating that both phase A and phase W have the same phase to neutral RMS voltage V. Call the phase angle between these two phases T. The voltage between the phases is then given by V * 2 * sin(T/2). So for the 180 degree phase difference we get V * 2 * sin(180/2) = V *2

In summary, can you mix devices in distribution boards (including consumer units)? Yes, you can. But you need to seek assurance from the manufacturer of the original assembly that the devices will be compatible, or conduct your own study to ensure the requirements are met. In the words of BEAMA, ‘The installer has responsibility to act “with due care”. If this is not done then there is a probability that, in the event of death, injury, fire or other damage, the installer would be accountable under Health and Safety legislation.’Knowing the right type of device to install is essential to protect equipment in all types of buildings, as is reflected in the evolving guidance. Now for something nifty: the _instantaneous_ voltages between points must always add up. If I measure 5V (DC) from A to B, and 6V (DC) from B to C then I will get 11V from A to B. If I measure 5V from X to Y and 6V from X to Z, then I will measure 1V from Y to Z. Finally , if I measure 5V from A to B, I will measure -5V from B to A. But this is all DC, or instantaneous voltage. Now imagine another point in the system, also connected to phase A. If we measure the voltage at this point, we will get the exact same curve. Finally, try to measure the voltage between these two locations. If you look at each instant in time, the voltage _difference_ will be zero. The average of zero is still zero. Net result is that if you measure the voltage between two points, both phase A, you will get zero volts, as expected. should be taken as soon as possible to improve the safety of the installation." would be appropriate but maybe your not talking about the LoadMaster Cor 40 O C , lhe purpose of the latter value being to avoid the necessity of de-rating thermaiiy sensitive circuit-

For this reason, regulation 536.4.203 states that ‘The relevant part of the BS EN 61439 series shall be applied to the integration of mechanical and electrical devices and components, e.g., circuit-breakers, control devices, busbars into an empty enclosure or existing low voltage assembly.’ Max Zs is the maximum permissible values of earth fault loop impedance (Ω), some maximum Zs values are listed in BS 7671, the maximum earth loop impedance values listed in the Zs tables are used to compare against the actual measured earth loop impedance values to ensure compliance with BS 7671.Which leaves you with the lighting circuits - how far down the cable do you need to go, if the 3kA value is present at the DB, using 1.5mm2 conductors before you are safe again - not far I suspect. I suspect that this latter condition will not be a problem in this case as this is for a specific fault current below the maximum that is already within the scope of the existing installation, so it will be sufficient to do a calculation for the increased let through energy with the new short circuit current. so that implies with that breaker 2.5mm is not OK, but you are comfortably OK with all cables of 4mmsq up and larger. How accurate is the 3kA value, and is that subject to cut off characteristics from upstream devices