on each telephone and on all the sockets, and hence the system was not that common. Sockets A domestic single British telephone line installation will have a single master socket or line box in the premises, which is provided by BT or another service provider: this socket is the demarcation point between the customer-owned and maintained on-premises wiring, and the telephone network. For installations using the NTE5 line box (NTE for network termination equipment), the demarcation point is actually within the socket: the lower half of the front plate and associated wiring is the customer's, while the permanent wiring on the non-removable section behind this, remains the responsibility of the service provider. Customers are not permitted to access the wiring in a master socket without a removable lower section. Plug-in extension kits are available for customers with this type of installation. The two wires from the exchange are denoted the B leg at −48V relative to ground when the line is not in use and the A leg which is near ground potential when the line is not in use. The A leg goes to pin 5 and the B leg to pin 2 in the master socket. (Although all equipment will work with a reversed line, so a reverse wired socket is not strictly a fault.) When current is flowing on the line, the B leg voltage collapses to nearer ground and the A leg voltage moves nearer to the B leg voltage. The exact voltage drop is a function of the distance to the exchange, and the network wiring type. According to SIN 352 the average DC current in the loop and voltage across the phone will be up to 42 mA at 12.5 V (short line), up to 33.5 mA at 10 V, and will be not less than 25 mA at 9 V. (long line limit) Line polarity reverses during calls if caller ID is in use. Once in a call the audio and tone levels superimposed on the DC voltage are expressed as dBm in 600 ohms, although the line impedances are permitted to be some way off 600 ohms (as per SIN351) -9 dBm (275 mV RMS) [0 dBm = 1 mW ( 0.775V RMS) in 600 ohms. Until recently, this socket contained an enclosed spark gap, SP1, that could safely flash over internally to provide high voltage surge protection. This component is no