Tech Tip 002

How do we combine MCB’s and RCBO’s?

In the outdoor lighting market in Australia the RCBO is now extensively used instead of the MCB. The MCB protects cables and electrical systems; the RCD protects human life. The RCBO is a device combining both MCB and RCD functions. These notes are intended to help better understand issues surrounding the selection process for these devices and is particularly applicable to LED lighting where Type 3 SPD’s are in wide usage.AS/NZS3000 the Standard itself does not require SPD’s to be installed. Appendix ‘F’ is provided for guidance purposes and therein notes that where an SPD is to be installed on the load side of an RCD, that RCD should have a ‘breaking capacity’ (Icon) of not less than 3kA. It continues with a recommendation to use S-type RCDs in accordance with AS/NZS61008.1 and AS/NZS61009.1.

IEC Standards stipulate that when placing an SPD on the load side of an RCD device the use of an S-type RCD in conformity with IEC61008-1 or IEC 61009-1 is required.

This device must withstand a 200A ‘ring wave’ impulse and have an immunity to a 3kA ‘impulse surge current’ (based on a standardised 8/20 μs current wave).

What is breaking capacity? It can be defined as the maximum level of fault current which can be safely cleared.

It is the highest fault current that the MCCB can trip without being damaged permanently. The device will be reusable after interrupting a fault, as long as it doesn’t exceed this value.

What is impulse surge current? It can be defined as current that rises and falls ‘sharply’ from the normal value and in a short duration of time. Devices will normally be rated as having an ‘impulse withstand’ rating.
Conclusion: The widely used 6A Clipsal RCBO (4RCBE206/30S40) has a ‘breaking capacity’ of 3kA which might suggest that it is suitable. However, It also has an ‘impulse withstand’ rating of 250A which according to AS/NZS61008/9.1 makes the unit unsuitable for use ‘upstream’ of an SPD.
Another way? Our approach is to place the RCBO ‘downstream’ of the SPD as recommended. However, the SPD still requires short circuit protection, so we place an MCB ‘upstream’ of the SPD device.

Clipsal have acknowledged this to be a suitable solution. However, when an RCBO must be installed ‘upstream’ of an SPD they have recommended the use of their A9D33620.