Editing Circuit breaker (section)

==Standard current ratings==
[[File:Standard Trip Characteristic of a Thermomagnetic Circuit Breaker.svg|thumb|Time till trip versus current as multiple of nominal current]]

Circuit breakers are manufactured with standard ratings, using a system of [[preferred number]]s to create a useful selection of ratings. A miniature circuit breaker has a fixed trip setting; changing the operating current value requires replacing the whole circuit breaker. Circuit breakers with higher ratings can have adjustable trip settings, allowing fewer standardized products to be used, adjusted to the applicable precise ratings when installed. For example, a circuit breaker with a 400 ampere ''frame size'' might have its over-current detection threshold set only 300 amperes where that rating is appropriate.

For low-voltage distribution circuit breakers an international standard, IEC 60898-1, defines ''rated current'' as the maximum current that a breaker is designed to carry continuously. The commonly available preferred values for rated current are 1{{Spaces}}A, 2{{Spaces}}A, 4{{Spaces}}A, 6&nbsp;A, 10&nbsp;A, 13&nbsp;A, 16&nbsp;A, 20&nbsp;A, 25&nbsp;A, 32&nbsp;A, 40&nbsp;A, 50&nbsp;A, 63&nbsp;A, 80&nbsp;A, 100&nbsp;A,<ref>{{Cite web |date=16 September 2016 |title=What is an MCB and how does it work? |url=https://www.fusebox.shop/pages/what-is-an-mcb-and-how-does-it-work |url-status=live |archive-url=https://web.archive.org/web/20180711194710/https://www.consumerunitworld.co.uk/what-is-an-mcb-and-how-does-it-work-328-c.asp |archive-date=11 July 2018 |access-date=11 July 2018 |website=Fusebox Shop }}</ref> and 125&nbsp;A. The circuit breaker is labeled with the rated current in [[ampere]]s prefixed by a letter, which indicates the ''instantaneous tripping current'' that causes the circuit breaker to trip without intentional time delay expressed in multiples of the rated current:

{| class="wikitable"
|-
! Type
! Instantaneous tripping current
|-
| B
|  3–5 times rated current {{var|I<sub>n</sub>}}, e.g. a nominally {{nowrap|10 A}} device will trip at 30–50 A
|-
| C
| 5–10 times {{var|I<sub>n</sub>}}
|-
| D
| 10–14 times {{var|I<sub>n</sub>}}
|-
| K
| 8–12 times {{var|I<sub>n</sub>}}
For the protection of loads that cause frequent short-duration (approximately {{nowrap|400 ms}} to {{nowrap|2 s}}) current peaks in normal operation
|-
| Z
| 2–3 times {{var|I<sub>n</sub>}} for durations on the order of tens of seconds.
For the protection of loads such as semiconductor devices or measuring circuits using current transformers.
|}

Circuit breakers are also rated by the maximum fault current that they can interrupt; this allows use of more economical devices on systems unlikely to develop the high short-circuit current found on, for example, a large commercial building distribution system.

In the United States, [[Underwriters Laboratories]] (UL) certifies equipment ratings, called Series Ratings (or "integrated equipment ratings") for circuit breaker equipment used for buildings. Power circuit breakers and medium- and high-voltage circuit breakers used for industrial or electric power systems are designed and tested to [[ANSI]] or [[IEEE]] standards in the C37 series. For example, standard C37.16 lists preferred frame size current ratings for power circuit breakers in the range of 600 to 5000 amperes. Trip current settings and time–current characteristics of these breakers are generally adjustable.

For medium- and high-voltage circuit breakers used in [[switchgear]], [[electrical substation|substations]] and generating stations, relatively few standard frame sizes are generally manufactured. These circuit breakers are usually controlled by separate [[protective relay]] systems, offering adjustable tripping current and time settings as well as allowing for more complex protection schemes.