Replacing a RCD - how to cut power?

[asktoby]

For a period of a few years we had some damp in the loft that I think was causing the our RCD to trip, sometimes multiple times a day.
(The leftmost RCD in this picture: HAGER 63 AMP 30mA DOUBLE POLE RCD TYPE AC U762 U2 20470.)

That damp issue is fixed and the RCD has stopped it's daily tripping, but I suspect it might be worn out from those many repeated trippings.

The symptom is that on some days it repeatedly trips when I try to start our treadmill, but on other days it's totally fine.

Acknowledging that it may be an overloaded circuit from the current draw of the treadmill, I'd like to fit a replacement RCD, just to rule out that the RCD is worn out.

I'm attaching a photo of the Consumer Unit, and the electrical box outside. What's the correct way to make the Consumer Unit safe to work on to replace the RCD?

 

[opps]

I AM NOT AN ELECTRICIAN

Just turn off the main switch on the far right and you can replace the RCD, but I am not sure that is the problem. If, for whatever reason, the treadmill has an excessive initial load, I would expect that to trip the MCB, not the RCB.

You need to wait for a proper electrician to advise you further, but I suspect that there may be residual moisture somewhere, perhaps use the treadmill plugged into an extension lead connected to a socket nowhere near the previously damp area and see how you get on.

 

[Harry Bloomfield]

I'm attaching a photo of the Consumer Unit, and the electrical box outside. What's the correct way to make the Consumer Unit safe to work on to replace the RCD?

You cannot, without pulling the DNO's cutout fuse. Ask your DNO if the might fit an isolator, next to the meter, sometimes they do this for free.

 

[Jurassicspark]

That damp issue is fixed and the RCD has stopped it's daily tripping, but I suspect it might be worn out from those many repeated trippings.

Not really.. you cannot assume things without testing.

The symptom is that on some days it repeatedly trips when I try to start our treadmill, but on other days it's totally fine.

Sounds like a N-E fault.

Acknowledging that it may be an overloaded circuit from the current draw of the treadmill, I'd like to fit a replacement RCD, just to rule out that the RCD is worn out.

The RCD won't do jack in this case.

I'm attaching a photo of the Consumer Unit, and the electrical box outside. What's the correct way to make the Consumer Unit safe to work on to replace the RCD?

Replacing the RCD without testing wouldn't be so wise to do.

I'd say get an electrician to replace the consumer unit, get the DNO to fit an isolator and get the circuit tested.

Your consumer unit raises a lot of issues.

 

[justigator]

Please do not be tempted to pull the DNO's cutout fuse yourself. Electricians sometimes do it - those (good, NICEIC-registered company) who replaced my consumer unit a couple of years ago did, before replacing the CU and installing an isolator after the meter for the next time - but they shouldn't. This was confirmed when I had a DNO bod visit later for another reason. Why not DIY? Well, even if there is no load from the installation, these things can be decades old and can disintegrate and if you are unlucky, arc between conductors protected only by the massive fuse somewhere down the road serving multiple properties. The DNO, at their request, changed the service head at my previous house. Full PPE including rigid face mask, thick rubber gauntlets and rubber wellingtons. Wooden stepstool. They allowed me to watch in fascination from the other end of the room. Not for me, ever. I am now 167 years old, give or take a century, and have every intention of staggering on un-electrocuted for a few years more.

 

[opps]

Again, I am not an electrician... I have never replaced a RCD, but I have replaced a few MCBs. I only ever turned off the main switch in the CU. I have never seen an electrician do more than that. Am I missing something?

I am not talking about removing the DNO fuse and fitting a new CU, I am referring to working on existing CUs.

 

[Murdochcat]

Why don’t you get a local competent spark to test the existing RCD properly?

Then you’ll know what you are dealing with

 

[Harry Bloomfield]

Again, I am not an electrician... I have never replaced a RCD, but I have replaced a few MCBs. I only ever turned off the main switch in the CU. I have never seen an electrician do more than that. Am I missing something?

The skill, knowledge, and experience needed, to know which bits are live and to be carefully avoided, plus how to minimise the risks.

 

[plugwash]

I'm attaching a photo of the Consumer Unit, and the electrical box outside. What's the correct way to make the Consumer Unit safe to work on to replace the RCD?

In an ideal world, one would isolate upstream before opening an enclosure. In practice this is often not practical. Customers are not supposed to pull the supplier's fuse and separate isolators only became the default relatively recently.

If the consumer unit is of a reasonably modern design, not damaged, was installed competently and you understand what is going on in there, it is reasonably safe to work in the CU with the main switch turned off but the incoming supply live.

If the consumer unit is damaged, was poorly installed, or you don't understand what you are doing the risk level rises dramatically.

So when opening a CU that you have not opened before and where upstream isolation is impractical, the procedure would go something like.

1. Inspect the CU from the outside. Does it look to be of a reasonably modern design? Are there any red flags (tape on the cover is not a great sign that the last guy knew what they were doing.....).
2. Switch off the main isolator.
3. Very carefully remove the cover. DO NOT stick your fingers inside while doing so.
4. Visually inspect the internals of the CU. Do the input terminals on the main switch look adequately guarded? where do the wires go? has poor termination left copper visible on the input connections? is there any evidence of multiple feeds?
5. Check for dead between all combinations of live/neutral/earth at the main switch output. Electricians would use a purpose made 2-pole tester for this and would "prove" it on a known live source (either a proving unit or the incoming terminals of the isolator) before and after testing.
6. Start work, carefully. Think about where you are sticking your fingers, use insulated tools.