From a BR training booklet:

Operation of AC8
AC8 alternators generate alternating current which is then fed to the RUG unit. The rug unit consists of the diode assembly (silicon type) which rectifies the output.

The rug unit also consists of a second diode assembly making the alternator self excited and controlling the rotor circuit to control the output of the machine.

Replacement of AC8 Alternators and RUG rectifier / regulators
The CAV AV8 Alternator (both 50 Amp - BR Cat No. 15/58501, and 75 Amp - BR Cat No. 14/94221) and their associated RUG units were gradually phased out when more than minimum repair was required at Works.

RUG units that were considered repairable were modified at Works to help reduce maintenance and improve battery charging at engine idling.

The modification consisted of fitting a new diode assembly making the alternator self-excited (similar to the AC203). This eliminated the alternator field relay (energised via the train wires 33 and 34 and the drivers key switch (see wiring diagram below). Battery charging could then take place with the engines idling without the need for the drivers key to be 'ON'.

Additionally, the main rectifying diodes (Germanium type) were replaced by the more reliable Silicon type with the removal of the two protection relays, test button and indicator lamp.

The modification was covered in Engineering Instruction MD321 issued to BREL. All modified units were designated by the suffix 'A' as follows:-

	Type	BR Cat No.	Battery Type	Voltage Setting
Original Modified	RUG 11 RUG 11A	14/94904 15/11281	Lead Acid	29.5V
Original Modified	RUG 14 RUG14A	14/98412 15/11285	Lead Acid	29.5V
Original Modified	RUG 15 RUG 15A	14/94812 15/11286	NIFE	31.5V

Modified and un-modified RUG units are NOT INTERCHANGABLE

Either
a) For an unmodified unit - cables 33 and 34 to be connected up to terminals 1 and 2

Or
b) For a modified unit - cables 33 and 34 to be left disconnected and taped up.

FAILURE TO DO THIS WILL DAMAGE THE ALTERNATOR FIELD DIODES AND CABLES

Modified units are identified by the removal of the protection unit (including the test button and indicator lamp).

During modification, terminals 1 & 2 (between B+ and B- in the D.C. connection block) have been rewired to allow for toilet water heating to be installed on certain power cars at works overhaul. It is therefore not possible to interchange unmodified and modified RUG units.

The following connections are correct:-

Refurbished Class 101 DMC(L) vehicles with toilet water heating
These vehicles are identified by a heater water control panel mounted next to the RUG unit.

Note - only RUG 11A units must be fitted
Terminals 1 and 2 to be connected up. (A full description of toilet water heating is in W.R. Standing Order 2/H/104).

 

Inside a RUG 11 unit