Restoring Power



I had a huge problem with ‘nuisance trips’ of the residual current device (RCD) in my house, which has been resolved by the installation of residual current circuit breakers with overcurrent protection (RCBOs). More reliable power to individual circuits in the house (and particularly the garage) has forced me to set up better monitoring so that I’m actually aware of circuit breaker trips.


I live in a new build house that was completed in 2002, but it has perhaps the least reliable electricity supply I’ve ever encountered. Brownouts and power cuts (short and long) are all too frequent, so I’ve invested in a number of uninterruptible power supplies (UPSs) to protect power to my home network and the servers (and their files) that live on it.

As part of the wiring regulations that existed at the time all of the power sockets in my house were protected by a single RCD in the consumer unit (aka ‘fuse board’). The RCD was suffering from an increasing number of ‘nuisance trips’ (activating when it wasn’t actually saving me or somebody else from electrocution), taking power to the entire house with it each time it went off.

Why now?

In the run up to Christmas I was experiencing more nuisance trips than ever – three in a single day at one stage; and every time we turned the oven on there was a minute of anticipation about whether it would trip the board again[1].

The incident that spurred me into action happened over New Year. We took a long weekend away with family and friends in the North East of England and Borders of Scotland. On returning home the house was freezing – the power had been out since the evening that we left, and so the central heating had been off. At least the cold weather meant that stuff in the fridges and freezers wasn’t impacted. It took hours to get everything back on – it seemed impossible to have all of the house circuits on at once without tripping the RCD and taking everything down again[2].


A bit of online research led me to this Institution of Engineering Technology (IET) forum thread about nuisance RCD trips in a house with ‘a lot of computers’, the response was:

Too many computers, cumulative earth leakage is tripping the RCD.
Ditch the single RCD and install RCBOs.

I’d not previously heard of RCBOs, but they’re basically a device that combines the functions of a Miniature Circuit Breaker (MCB) and an RCD into a single breaker. This has a couple of advantages:

  1. When a trip happens due to high residual current leakage to earth it only takes out a single circuit rather than the whole board.
  2. Any background levels of current leakage get spread across a number of breakers rather than accumulating on a single breaker and bringing it ever closer to its tripping point.

I got in touch with a friendly electrician[3] and he’s now installed a new consumer unit along with RCBOs[4] for each of the socket circuits (and in accordance with updated wiring regulations the light circuits are now RCD protected too using a ‘split board’ approach).

Since getting the RCBOs installed there hasn’t been a single trip on any circuit – so mission accomplished :)

A quick detour on suppression

The IET Forum Thread I referenced above also contains some discussion of the problems that come with transient voltage suppressors (TVSs). The issue is that when a there’s a voltage spike the transient suppressor gives it a path to earth, which then causes a residual current leak that trips an RCD.

I removed all of the surge suppressed power strips from the house, but it didn’t make any difference, likely because I left in place my 4 UPSs, which I expect all have their own TVSs within.

Every silver lining…

Now that I had reliable power there was a new problem. How would I know when power to the garage (and the server and freezer out there) went off? A trip on the garage RCBO wouldn’t take out the rest of the house, so it would be far less obvious. If I was away on a business trip it’s possible that the rest of the family wouldn’t notice for days.

The answer was to have better monitoring…

Time to go NUTs

I have a PowerWalker VI 2200 out in the garage looking after my Dell T110 II and associated network switch. Like most UPSs it has a USB port that provides a serial over USB connection. I hooked this up to a VM running on the server and installed  Network UPS Tools (NUT) to keep an eye on things.

Email alerts

I use upsmon to run upssched to run a script that sends me an email when power goes out (and another when it’s restored). I’ve dropped the configs and scripts into a gist if you want to do this yourself (thanks to Bogdan for his ‘How to monitor PowerMust 2012 UPS with Ubuntu Server 14‘ for showing me the way here)

Graceful shutdown

My NAS and Desktop NUC both interface directly by USB with their local UPS and are configured to shut down when the battery level goes critical. I needed the same for the VMware vSphere servers running off the other two UPSs (in my garage and loft). Luckily this has been taken care of by René with his ‘NUT Client for ESXi 5 and 6‘ (Google Translation to English) so I just had to install and configure that, pointing at the same NUT servers I used for email alerts.


RCBOs have totally solved the problems I was having with RCD nuisance trips, and I now have monitoring and graceful shutdown in place for when there are real power issues.


[1] Conventional fault finding would suggest a fault with the oven, but I wasn’t convinced given that it’s only about 18 months old. My take was that a trivial issue with the oven that wouldn’t trip the RCD on its own was taking it over the edge when added to other leakage on other circuits. The fact that the oven doesn’t trip its new RCBO seems to bear that out.
[2] Going through the usual fault finding to identify a single culprit was fruitless. Everything was wrong and nothing was wrong. It didn’t matter which circuit I isolated. My hypothesis became that there was probably about 3-4mA of leakage on each circuit in the house, combining to a total leakage that had the single RCD on a hair trigger of any additional leakage.
[3] Despite being an electronics engineering graduate, a member of the IET (MIET) and a Chartered Engineer (CEng) I’m not allowed to do my own electrical work. I recall that there were ructions about this when the revised regulations with proposed and introduced, but it’s pretty sensible. Messing around with the 100A supplies to consumer units is no business for amateurs, and I was happy to get in a professional.
[4] After a bit of hunting around and price comparison I went for a Contactum Defender consumer unit and associated RCBOs and MCBs from my local TLC Direct. The kit came to just over £200, and getting it professionally installed cost about the same – so if you need to do this yourself budget around £350-500 depending on how many circuits you have.

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