Installers of permanent electrical wiring are required to ensure that new work meets current regulations. The UK wiring regulations have been converted to a BS specification (BS7671) which at the time of writing is being CENELEC harmonised. Current regulations are designed to ensure that new installations are safe under fault conditions. `Safe’ means that the installation cannot harm a user OR cause a fire. For normal industrial and domestic site work the required testing usually covers BOTH needs.
In some special cases where the wiring is modified by local conditions EXTRA tests and results must be considered before a site can be signed off by the installer. There may also be situations where commonly used test equipment may damage part of the installation and defeat one or more reasons for the test. As manufacturers of electrical power conditioning equipment we offer the following guidelines to the extra special conditions which must considered.
This may be an in-line filter or a generator. The filter may be of a galvanically isolated type. The generator may be a small rotary type or the static inverter found in uninterruptible power supplies (UPS).
The wiring regulations and good practice are founded on the expectation that electrical power is derived from a low impedance source. Fault conditions usually result in the clearance of a protective element such as a fuse or magnetically operated breaker. Most power conditioning equipment is by it’s very intention NOT low impedance.
In addition there may be electronic or magnetic control of either voltage or current which cause the equipment to have a real impedance which is modified in operation to an apparent impedance. The installer must be aware of the critical schematic for the equipment being installed and consider all operational modes of current path selection devices. In addition distribution discrimination must be shown in the usual way.
Two different views must be taken in cases where the supply is provided by an apparent or actual higher impedance source:
The first consideration is user safety:
the installer must ensure that when any live conductor is connected by a fault to an exposed earth that the resultant VOLTAGE is not unsafe. (In practice it is assumed that the fault is zero impedance) The voltage generated across the earth impedance (Ze) by the fault current must be `safe’. Although this is usually below 50 volts good practice and margins mean that a target of 5 volts is more realistic. If there is significant source impedance (Zs) whether real or apparent this must be considered in the calculation of the worst case fault current.
The second consideration is fire:
the installer must consider the worst case CURRENT under any one fault condition. If the apparent source impedance of the supply (shown as Z) is large enough to prevent the wiring from overheating then the protection is NOT required to open.
|
|
Since there is an almost infinite variety of potential installation variations even using our own products some further suggestions for ensuring safe installation are outlined in hb067
.
| |
 |