RCBO Failure

[John Carroll]

 

Stayed in daughters house at weekend, had a shower on Saturday, checked trip switch on the RCBO a few hours later, RCBO tripped & reset . Got a call on Sunday morning, no power on shower, found no continuity between the RCBO live contacts, renewed RCBO. Checked shower power at 8.8kw (9.0kw shower) using the smart meter flashing LED. Broke open the failed RCBO (Hager 40A/30ma) and found signs of overheating, the contact (shown below) closes downward, there is a layer of what looks like melted plastic as shown by the arrow which was attached underneat the contact as well which probably prevented the contact closing or it closed with this plastic insulation preventing continuity, I get OC on the M.Meter when testing from the blob of plastic to the contact, also signs of overheating inside the RCBO.

I wonder what would have happened if I hadn't tested the RCBO?, I do test it ~ once/annum, this was the original RCBO, installed when the house was built in 2006.

The shower is switched on/off at the shower, then isolated each time with the pull cord, this RCBO has never opened/closed on load, is this a unusual failure?

Edited March 6 by John Carroll

[Alan Ambrose]

Could be overheating failure I guess as it's running at about 38A vs. max of 40A if your numbers are correct. That's a lot of power. These days the cables in the consumer unit should be torqued using a proper torque driver. So, it's also possible that a slightly loose-ish connection has been arcing.

[John Carroll]

What ever about torqueing, the screws were certainly well tightened as were all the other CU cables.

A 40A RCBO should be well able IMO to carry 40A, a type B  can carry 1.2/1.25 times rated current (~ 50A) for hours on end, > 10, 000 secs according to the charts. My own "230V" supply was giving 240V at the (running) 9.0kw shower terminals for about 10 years before returning to normal so I had a lovely 9.8kw shower drawing nearly 41A, the same B40 RCBO is the original, installed in 2007.

 

 

[Alan Ambrose]

Well there’s are so many ways in physics to generate heat from electricity so suggest one of those mechanisms are involved. If you can get over the initial shaky camera and compulsive mcb switch operation this is interesting:

 

https://youtu.be/gOUSDDLKICk

 

Mike is always good value on this type of investigation. I had no idea how complex RCBOs are both electrically and mechanically. Which begs the question what’s inside more complex devices llike afdds etc.

[Mattg4321]

Unusual for them to fail like that, especially Hager ime. It won’t have been installer error as that contact has nothing to do with the terminals top or bottom.  Sounds like it’s been running under its rated current too, albeit only slightly. 
 

Replace and move on!
 

 

[John Carroll]

On 07/03/2024 at 06:43, Alan Ambrose said:

Well there’s are so many ways in physics to generate heat from electricity so suggest one of those mechanisms are involved. If you can get over the initial shaky camera and compulsive mcb switch operation this is interesting:

 

https://youtu.be/gOUSDDLKICk

 

Mike is always good value on this type of investigation. I had no idea how complex RCBOs are both electrically and mechanically. Which begs the question what’s inside more complex devices llike afdds etc.

 

Very interseting Video, that's certainly a lot of safety gear for a relatively cheap purchase price.

 

 

 

 

 

 

 

Edited March 9 by John Carroll

[SteamyTea]

On 07/03/2024 at 06:48, Mattg4321 said:

Unusual for them to fail like that, especially Hager

Could be sent to them, they are probably interested.

[TonyT]

Think they industry has a 7% failure rate on RCD/RCBO I remember from BEAMA documentation

[SteamyTea]

2 hours ago, TonyT said:

Think they industry has a 7% failure rate on RCD/RCBO I remember from BEAMA documentation

That is quite high.

 

This article seems to think the failure rate is 3%, still high in my opinion.

https://www.electricalsafetyfirst.org.uk/guidance/safety-around-the-home/rcds-explained/

 

 

Edited March 9 by SteamyTea

[Mattg4321]

Not in my experience. Nothing like that high. Perhaps that’s under certain circumstances. Like when they are run continuously at full rated current or something. Failed inside what timeframe?

 

I fit hundreds of RCBO’s every year. Probably not far off 1000. I probably get 1 or 2 failures a year, even including devices fitted years previously. Admittedly though most people don’t use the test button! So if you’re including failure to operate under fault conditions then the figure will be higher. I assume you guys are thinking of total failure ie no power to circuit. 
 

As above it’s different if they are subjected to substantial current flow. The reality is that the vast majority are only having to deal with 1 or 2 amps at the most either all the time or nearly all the time. 
 

Electric showers and EV chargers are the biggest culprits for causing RCBO failure. 

Edited March 10 by Mattg4321