One thing I like about the NEMA flat-blade configurations is the interchangability where appropriate.
For example, a 5-15 receptacle will accept a 5-15 plug or a 1-15 plug (either the polarized or the non-polarized variety) A 5-20 receptacle will accept the appropriate 5-20 or 1-20 plugs, and the 5-15 and 1-15 types as well. Ditto for the 6-xx 240V versions.
Under the old round-pin BS configurations there was no such compatibility. We had both 2-pin non-grounding and 3-pin grounding versions of 5-amp plugs, for instance, but a 2-pin non-grounding 5A plug would not fit a 3-pin grounding 5A receptacle.
To complete the picture on the IEC309 connectors, the other two most commonly used configurations are:
2P+E 240V, used widely in industry for general-purpose single-phase loads, and the 16A version is virtually universal for campsite electrical hookups for RVs and travel trailers:
2P+E 110V, used for our low-voltage building site power tools on a center-tapped 55-0-55V system:
[This message has been edited by pauluk (edited 10-18-2005).]