The IET's "electricians rulebook" BS7671 mentions the use of these types of devices to protect both the CU devices and electrical appliances from surges induced by lightening. Manufacturers of consumer units with SPDs need to comply with BS7671 534.4.6

Why would this be particularly relevant to renewable energy, you may ask?

When installing a renewable energy additions like a battery storage system or solar PV, consumers may want to ensure the rest of their electrical installation is robust to cope with current risks and new ones related to the introduction of the devices. Often they will use the opportunity to replace their CU, despite it being strictly compliant or not.

@transparent suggested this deserves a separate thread. 

So there are different views about how a CU manufacturer is expected to provide the SPD capability. @transparent shared this discussion on IET´s forum about this topic. 

A key question is whether the SPD device should be protected by an MCB (and if a CU with an SPD protected by a standard MCB is providing adequate protection from surges). 

Some RHH forum members shared examples of different designs of SPD CUs:

• The SPD in my consumer unit (Fusebox pink SPD) is direct to busbar, relying on the DNO fuse for protection, the previous model (Fusebox orange SPD) required MCB protection.
• The question of an MCB upstream of an SPD has always intrigued me but reading up on the topic has given some insights regards how the 'instantaneous' trip mechanism of the MCB might deal with high frequency, high voltage transients.
• The main 100A CU we had installed about a year ago (from Contactum) does not have an MCB protecting the SPD. But the SPD is next to the main CU isolator!

Worth reproducing a photo of a consumer unit from a forum member to exemplify that "questionable" design.  

Below I reproduce a description to help anyone less familiar with it. 

The 100A Main Switch is on the right, correctly labelled.

It supplies a row of 18 RCBOs. Each RCBO provides two types of protection to the circuit it feeds:

• over-current protection; just as with a fuse, the circuit gets switched off if too much current is drawn
• earth-leakage protection; if the current returning on the neutral wire is 30mA (or more) different to that being supplied on the live wire, the circuit switches off

At the far left-end of the Consumer Unit is a yellow anti-surge device.

I could ask some questions about the design but more experienced members would do it better! 

My post is also looking to: 

• help future readers that discover that the CU in their home has this type of design decide what they can do.
• identify anything that could be done to enable and influence improvements in CU designs currently in the market. 

My thanks also to @trebor12345 @ga3_usr @jancold for their contributions