Electrical Supply Characteristics

Characteristics of Electrical Supply

1. It is a requirement of the IET Wiring Regulations that the designer shall determine the characteristics of the supply.

2. Some of the information must come from the Electricity Supplier (E.S.) and he has a Statutory obligation to provide it under the Electricity Supply Regulations. These include:

a. voltage, frequency, number and rotation of phases;

b. maximum prospective short-circuit current at the supply terminals (Ip);

c. maximum earth fault loop impedance external to the installation (Ze);

d. the type and rating of the E.S's protective device nearest to the supply terminals (origin of the installation).

3. To determine the suitability of the supply for the requirements of the installation the designer must provide information to the E.S, including:

i) maximum demand; Any provisional MD must be confirmed prior to committing the Client to an agreement with the ES as the MD charge is likely to be enforced for a 5 year agreement period.

ii) nature of the load, highlighting unusual characteristics such as equipment with high inrush currents or high harmonic content.

02 Voltage and Frequency:

1. The 'declared' supply voltages in the UK are: low voltage (LV) 400v 3 phase and 230v single phase; high voltage (HV) 11kV (there may be a few places where the HV supply is 6.6kV); for very large loads a higher supply voltage may be provided.

2. Frequency in the UK is 50Hz.

03 Choice of Supply Voltage:

1. The choice of supply voltage will be influenced by the load and possibly the location of the site. See also procedures 4 & 5 of Part 2.

2. Generally the E.S. will decide what the supply voltage will be, but where their selection is not in the Client's best interest we must negotiate for the preferred alternative. The selection may vary between different E.S.’s and will also depend on the capacity of the local network.

3. Where the MD, including agreed allowance for future, is less than about 500kVA (under 800A) it is generally preferable to have the supply at LV. This avoids the Client having responsibility for an HV installation. If the supply can be provided from the local LV network this may save the Client providing space for the E.S.’s switchgear and transformer.

4. For an MD greater than about 500kVA the E.S. will probably insist on an HV supply.

04 Security of Supply:

1. If we ask the E.S. for a higher security of supply than that given by his standard arrangement this will probably cost the Client a significant amount. Therefore, it is essential that the need for high security is carefully examined.

2. The supply system in the UK has a relatively high level of security, and should be adequate for most commercial projects provided they can survive an occasional outage. It is generally not economic to spend significant sums on duplicate supplies or 50-100% standby generation to cover such failures.

3. Security is greatest in large conurbations, less in rural or remote sites..

4. When a Client requires high security of supply this can be met to varying degrees by duplicate feeders from the E.S. However, the E.S. cannot guarantee no outage under any circumstances; e.g. major failure due to exceptional weather or severe industrial action. Therefore, the better way to ensure security is usually by means of standby generation for essential load plus UPS if necessary. In this case it may not be worth spending additional money on enhanced security of the mains supply. The costs of the alternatives, including maintenance and depreciation of generators, must be evaluated and a report given to the Client.

5. Supply arrangements and their reliability can be broadly classified as follows:

a. LV single feeder tee connected to external network: minimum security against local network failure especially if main is radial; if on an inter-connector between two sub- stations some protection is gained against equipment failure; no security against sub-station failure.

b. LV dedicated feed(s) from adjacent or dedicated sub-station: increased security against LV fault, especially if more than one LV feeder with bus-section switch on Consumer's switchboard. Substation almost certainly connected into an HV ring which may give some protection against equipment failure, see item d. below.

c. HV single feeder tee connected to external network: minimum security against local network failure especially if main is radial; if on a ring supply may be reconnected once the fault is found and isolated, unless fault is on the section to which the feeder is connected; limited security against sub-station failure, depending on how the ring is connected.

d. HV RMU connected directly into a ring main: re-connection can usually be made within about an hour by switching out the faulty section of the ring; if each end of the ring connects to separate 'halves' of the sub-station this protects against local equipment failure.

e. Two HV feeders each connected to a different ring main: this normally will cost extra unless the E.S. want flexibility to transfer load; if ring mains originate from the same sub-station there is only very little improvement on option d. Some benefit is gained if auto-changeover is supplied, but again this will cost money.

f. HV supply from local ring main with an independent feeder from a different sub-station: if the second sub-station is fed from another part of the 132kV or higher voltage system then considerable increased security is achieved against local failures; however, it would be necessary to understand the E.S. network to know how much extra security is gained for probably a large extra cost.