Safe Installation + BS 7671


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.

Power Conditioning Equipment

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.

Special considerations

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.

Safe UPS Installation + BS 7671


A typical uninterruptible power supply ( UPS) requires EXTRA calculations to be performed before installation and sign-off.

In the illustrated case the UPS has no galvanic isolation and the earth + neutral go through the unit. The recommended bypass switch means that it is possible for the distribution wiring to see the low impedance mains supply.

For the first part of the calculations the UPS should be ignored and the wiring calculated out in the normal way. It is unlikely that when the UPS is introduced into the system that the rating of any wiring will have to be increased EXCEPT the input supply.

Input supply

Most UPS equipment is not 100% efficient. In addition to supplying the full rated load current modified by the UPS efficiency there is a need to support the battery charger as well. The UPS manufacturer should provide worst case input power needs. For longer supply runs this often means that a larger wire is required than expected for an installation where there is no UPS.


For the first part of the testing the UPS should be isolated and the distribution connected to the main supply via the bypass switch. Normal checks should be applied using calibrated equipment.

NOTE: some modern pulse-based testing equipment can damage electronic equipment which should normally be disconnected during testing.

The second consideration is where the distributed power is supplied by the static inverter in the UPS. This has real and apparent impedance. The apparent impedance is electronically controlled and should be specified by the UPS supplier as a worst case current available from the UPS.

For user safety the installer must ensure that the earth loop impedance is sufficiently low so that it provides a voltage inside regulations for the worst case fault current available from the UPS. It should be appreciated that under these conditions over current fault protection may not clear.

For fire safety the installer needs to evaluate that all the distribution wiring will not overheat or it must clear protection under fault conditions. If the current is below the continuous current rating of the wiring the apparent source impedance may mean that the protection does not clear.