Ah.... The good old British ring circuit, although in my not-so-humble opinion the "good" part is highly debatable!
Yes, the ring circuit is still widely used over here. It was introduced along with the BS1363 (fused 13-amp) plug in the late 1940s.
Originally the cable size normally used was 7/.029, then with the change to metric 2.5 sq mm, which is about 20% larger than #14. Protection is then provided at the panel by a 30A fuse or C/B (32A on newer breakers).
The fuses used in the plugs are ceramic-bodied types 1-inch long by 1/4-inch diameter, available in various sizes up to 13A.
I read that to "tap" off of a ring circuit you must use a device called a "spur" unit.. I am assuming this device is a form of terminal block that also contains fuses or a breaker of some sort to protect the tap or "spur"???
There can be either a fused or an unfused spur. A single outlet can be run as a spur from the ring unfused. The same 2.5 sq. mm cable is usually used, which means that the 30A protection at source exceeds the rating of that spur cable. However, because all the plugs which may be connected to that spur outlet contain a fuse (max. 13A), the cable should, in theory, be protected.
The spur may be tapped from the ring either by using a regular junction box at some point, or by joining it at an existing outlet on the ring. The spur can even be wired directly fro the fuse/breaker and neutral bar at the panel.
In older installations, an unfused spur was allowed to feed two
separate outlets. It was considered that diversity would prevent excess load on the spur cable.
A fused spur unit, or "fused connection unit" is used to provide a fused spur. These units take the same BS1362 fuses as are used in the plugs, and are commonly employed to feed a fixed appliance from the ring (small water heater, pump, etc.). When fitted right next to the appliance, the FCU basically just provides a hardwired equivalent to a plug-&-socket connection.
A fused spur unit can also be fitted on a ring to feed fixed wiring of a smaller size. A typical use would be to feed a few lights via a 3 or 5A fuse.
Is an RCD the same kind of idea only its a 33 milliamp device that is used at the main and protects the entire dwelling??
Yes, and RCD is similar in principle to a GFI. The RCD (Residual Current Device) was formerly known as an RCCB (Residual Current Circuit Breaker), and originally known as an ELCB (Earth-Leakage Circuit Breaker).
American/Canadian trip levels are used only for special locations though. In normal domestic work an RCD with 100 or 30mA trip is currently the norm. Individual breakers which combine overcurrent and RCD features for protecting a single circuit are coming into use very gradually, but a large RCD which protects either the entire installation or a part of it is the norm.
We have several different grounding arrangements in use, and where the system known as TT is employed, an RCD is essential to provide ground fault protection to the whole installation.