Every item of equipment shall comply with the appropriate British or Harmonized Standard.’ For distribution boards, this is the BS EN 61439 series. Part 3 is a specific part of the BS EN 61439 suite and it includes requirements for Distribution Boards intended for use by ‘ordinary persons’. All SPDs work by diverting surge currents to earth to reduce the over-voltage to a level that will not damage the components of the system or equipment connected to it. There are three types of surge protection devices: Type 1 How accurate is the 3kA value, and is that subject to cut off characteristics from upstream devices Now, imagine that I start delaying phase W more and more. As I do so, it gets more and more different from phase A. The sine wave that is the difference between phase A and phase W gets larger and larger. Eventually this resultant sine wave is actually _greater_ than phase A. This difference sine wave reaches its peak when phase W is exactly 1/2 cycle from phase A, meaning that the +peak of A corresponds to the -peak of W. At this point, the amplitude of the _difference_ between the two phases is twice the amplitude of each phase alone. This is to say that if phase A is 277V relative to neutral, and phase W is 277V relative to neutral, and 180degrees out of phase with phase A, then the voltage A to W is 534V. Indeed as Mike mentioned 10ms gives a very large number (for 3kA and 10ms -->> energy let through is 90000).

Of note here is that a protective device may conform to the appropriate product standard (e.g. BS EN 60898) and be UKCA marked. If the devices are from different manufacturers, they are likely to be designed differently and will perform differently – though both may comply with the appropriate safety requirements of BS EN 60898. When purchasing a consumer unit or distribution board, the manufacturer’s instructions will state which devices can be used in the unit – and don’t forget according to BS 7671 (regulation 510.3) there is an obligation on designers and installers to follow the manufacturer’s instructions when selecting and erecting equipment. Make a graph of the difference between A and W, and you will find out that it is a sine wave with low amplitude. In fact, it is a law of mathematics that the sum or difference of two sine waves of the same frequency (but possibly different amplitudes or phases) is another sine wave of the same frequency, again with different frequency or phase. So I2t is actually a function of the prospective fault current, and is either depicted graphically, or listed as spot values . For mcbs made to the IEC standards since 1999 or so this is a spec parameter, and guaranteed by design. for earlier devices it isn't so that implies with that breaker 2.5mm is not OK, but you are comfortably OK with all cables of 4mmsq up and larger.So now consider phase A in our 277/480V wye system. We can plot the voltage relative to our earth reference as a function of time, and get a graph, ideally a nice sine curve. At time zero the voltage will be zero. At 1/240 of a second, the voltage will be +392V (277V * 1.414, the square root of 2). Then at 2/240 second the voltage will again be zero. At 3/240 of a second the voltage will be -392V, and at 4/240 second (1/60 second) the voltage will again be zero. This cycle will repeat. 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. The 17th edition of the IET wiring regulations amendment 3 introduced the Cmin (0.95) factor which reduced the old maximum zs values to allow for the fluctuation of the voltage. The Cmin factor is applied to the voltage to earth (Uo), so if the voltage is 230 the calculation would be 230×0.95 = 218.5 volts.

Step 2 remember that for AC circuits the voltage is a constantly changing value, and when we give a single number, it is a form of average voltage, called the RMS voltage. The RMS voltage of an AC waveform corresponds to the DC voltage which would deliver the same power to a resistive load. If you take AC at 480V RMS and apply it to a 480 ohm resistor, the average power delivered to that resistor would be 480W. If you take DC at 480V and apply it to a 480 ohm resistor, the power delivered to that resistor would be 480W. The values of earth loop impedance shown in these tables must compensate for conductor temperature rise, if the measurement of loop impedance is taken at ambient temperature. A useful rule of thumb is to allow for a temperature rise from 20 degrees to 70 degrees by multiplying the listed value by 0.8. The measured value can then be compared to the compensated value.If devices from different manufacturers are used together, the venting characteristics may not be coordinated which could result in significant further damage to adjacent devices or other parts of the distribution board. is a nice number for rules of thumb , 1.6mm is nearly 1/16 of an inch and almost exactly the diameter of 16 standard wire gauge and 16 Birmingham sheet metal gauge, though not american wire, but AWG 14 is ~ 1.6mm dia. ) One of the more common types of BS 3871 circuit breaker found on installations is the Crabtree C50 which was produced in single phase and 3 phase versions. These are all Type 2 circuit breakers and their fault current rating can be determined by the infill paint in the operating handle. White paint infill indicates a 3kA rating and yellow paint indicates a 4.5kA rating.

A Type 2 device offers protection against over-voltages from switching and indirect lightning strikes. This type more commonly uses a metal oxide varistor (MOV) to divert the current away. Type 3

is the old British standard for MCB's so BS 3871 does not tell anyone what type of MCB it is. I looked at http://www.beamainstallation.org.uk/assets/pdfs/CircuitBreaker.pdf which explains how the standards evolved. 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.’