Devices such as switches, RCCBs etc. in distribution boards and consumer units may have historically had their rated current determined after having taken diversity into account but without having considered overload protection of the devices.
These devices do not provide protection against overload and the 18th Edition prescribes that overload protection of the switch or RCCB shall not solely be based on the use of diversity factors of the downstream circuits.
Methods of protection
We recommend one of the following methods of protection against overload:
Ensure the sum of the rated current of the downstream MCBs do not exceed the rated current of the switch or RCCB (Inc). This method would however need to consider the consequences of any spare ways and later additions.
A consumer unit with a rated current of 100A (InA), with two RCCB 100A (Inc) will allow the consumer unit to be installed in any single phase application up to 100A
Ensure that the rated current of a switch or RCCB (Inc), stated by the assembly manufacturer, is not less than the rating of the upstream OCPD. For a domestic installation this could be a 100A cut-out fuse.
Potential future loads on spare ways should be considered
Select a consumer unit or distribution assembly that only utilises RCBOs on outgoing circuits. Consideration will still need to be given as to the rated current of the main switch.
What the Regs say
"RCCBs & switches do not provide protection against overload, therefore they shall be protected by an overcurrent protective device."
“ ... overload protection shall not solely be based on the use of diversity factors of the downstream circuits. To achieve overload protection of RCCBs or switches, the rated current of the overcurrent protection device (OCPD) shall be selected according to the manufacturers instructions”.
Overload Protection Files
Download our documentations below, containing everything you need to know about Overload Protection and the 18th Edition.
Overload Protection | White Paper
Our white paper on Overload Protection walks through the thinking behind choice of RCCBs in consumer units.
At a Glance | Overload Protection
All of the key info you need to know about Overload Protection of RCCBs and Switches.
Bitesize Guide | 18th EditionOur small guide to everything you need to know about the 18th edition.
Residential Distribution CatalogueOur Residential Distribution Catalogue contains all of our 18th edition solutions for residential applications.
More from the 18th Edition
A transient overvoltage or surge is a short duration increase in voltage measured between two or more conductors. In short this means anything from microseconds (millionths of a second) to a few milliseconds (thousandths of a second) in duration.