Electrical properties

In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer. This arrangement is a current standard for residential and industrial electric systems particularly in Europe.

The conductor that connects the exposed metallic parts of the consumer's electrical installation is called protective earth (PE). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral (N). Three variants of TN systems are distinguished.

TN−S: PE and N are separate conductors that are connected together only near the power source.

TN−C: a combined PEN conductor fulfils the functions of both a PE and an N conductor (on 230/400 V systems normally only used for distribution networks).

TN−C−S: part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, and earth and neutral are separated in the service head. In the UK this system is also known as protective multiple earthing (PME); similar systems are designated MEN in Australia and New Zealand and MGN in North America.

It is possible to have both TN-S and TN-C-S supplies taken from the same transformer. The PEN must be suitably reinforced against failure, as an open-circuit PEN can impress full phase voltage on any exposed metal connected to the system earth downstream of the break. The alternative is to provide a local earth and convert to TT. The main attraction of a TN network is that the low impedance earth path allows easy automatic disconnection of supply (ADS) in the case of a line-to-PE short circuit, and an RCD is not needed to detect earth faults.

NYM-J cable is PVC insulated, PVC sheathed, grey, 300/500 V rated to BS EN 50265-2-1.

NYM-J is suitable for industrial or home applications and designed for dry, moist or wet open areas, i.e. under plaster and in concrete — not suitable for exposure to direct sunlight.

NYY-J cable is a non-armoured mains cable that can be installed indoors where there is little chance of mechanical damage. It can be installed in open air, underground, in water and also in brickwork and concrete, with the exception of shaken, vibrated or compressed concrete.

The cross sectional area is defined as the square of the wire's diameter in mils, giving the area in units of circular mils. This makes number handling much easier.

If checked, uplan will draw the incoming cables in the form of a terminal block.

If checked, uplan will calculate and draw a service socket outlet in the electrical cabinet. This device can be useful during maintenance or programming of the BMS.

A main switch is a central cut-off switch that controls the smaller cut-off switches and machines of a building.

A surge protector (also spike suppressor, surge suppressor, surge diverter, SPD or TVSS) is a device intended to protect electrical devices from voltage spikes in alternating current (AC) circuits. A voltage spike is a transient event, typically lasting 1 to 30 microseconds, that may reach over 1,000 volts.

Lightning that hits a power line can give a spike of over 100,000 volts and can burn through wiring insulation and cause fires, but even modest spikes can destroy computers, battery chargers, modems and TVs that happen to be plugged in at the time. Typically the surge device will trigger at a set voltage, around 3 to 4 times the mains voltage, and divert the current to earth. Some devices absorb the spike and release it as heat; they are generally rated according to the amount of energy in joules they can absorb.