In South Africa, most plug tops are unfused and are rated 16A. I would like to know how they determine what current value they'll rate a plug, and what factors influence how they rate a plug when designing it.
I know some countries use the 15A and 13A, but surely the exact value was calculated? How was this done because the information online says nothing about why the plug is rated the way it is.
This is a 'rating'.
A committee somewhere has decided on some (reasonable we hope) standard test conditions, and a limiting temperature for the plug elements. 16A is the current which achieved that temperature under those conditions.
This obviously begs the question of why those conditions, and why that temperature.
Keep in mind that (particularly with the fat "M" South African plugs) the plug might actually be safe at 30A or 40A rather than 16A, but 16A is the highest useful rating (because of wiring or other limitations such as the contact pressure in the female socket), so that's all they bother to test for.
(The M plugs are sort of like a round-pin version of the huge UK mains plug and are enormous in comparison with most other mains plugs used worldwide).
As @Neil_UK says, the limitations are a product of a committee that comes up with requirements for safety including temperature rise, fire retardant characteristics, and shock safety and markings. The requirements are typically codified into a general law requiring conformance (National Regulator for Compulsory Specifications Act) and the actual technical testing requirements are determined by specified groups (and revised from time to time, without there necessarily being changes to the law). The actual testing can be done by whichever laboratories are accredited, for example, a UK laboratory could be used, and they will test to the required standard laid out by the South African authorities.
In this case, it appears that the relevant standard only applies up to 16A, so there is no point in testing for more.
The requirements may seem a bit arbitrary, but in virtually every case there will be sound reasons why every one of the requirements are in place, and they will be agreed to by industry and government representatives with the aim of assuring safety yet not making the product too difficult or expensive to produce (and sometimes as a non-tariff barrier to protect local companies, but that's another story).
In North America, the Electrical Code specifies different plug configurations for different voltage and current ratings, to prevent incorrect connections between device and supply. The actual current and voltage capability of these connectors may have little or no relation to the Code-specified ratings.
For example, there are different contact arangements, but otherwise apparently identical construction, for plugs rated by code for 125V/15A, 125V/20A, 250V/15A and 250V/20A. These connectors differ only in the orientation of the contact blades, so all should be safe to use on a 250V/20A circuit, but the Electrical Code says only the 250V/20A configuration can be used on a 240V, 20A circuit.
I expect a similar situation would occur in most other jurisdictions.
