Are all RCBOs equal?

[seasickstevie]

Obviously not ... but any opinions on how CTI (Wylex generic NH fit) RCBOs might compare quality wise with the more expensive Wylex NH product?

Going in a Wylex CU.

Big difference in price - Wylex being nearly double the price of CTI.

 

[DaftPunk]

If you've got a wylex cu then you MUST use wylex devices in it. Mix and match within a cu is not acceptable.

 

[ericmark]

As stated for a consumer unit to be a consumer unit it has to be type tested which means you can only use manufacturers recommended products for that unit otherwise it becomes a distribution unit and there may be issues where it is used by an ordinary person the same now it would seem applies to some RCD's which amendment 1 of BS7671:2008 has said should not be used by ordinary persons these

Untitled

• ericmark
• 6 Dec 2008
• 34

would auto reset themselves but no longer allowed in a house not that at £350 each you would want one.

Some RCD's like the X-Pole have a monitor built in to tell you where the leakage is approaching the point where it is likely to trip and trip at 90 - 100% of rated trip current but most are rated to trip between 50 - 100% of rated trip current.

There are also types AC, A, and B with inverter supplies using simulated sine wave we have to use A type but in the main we use the AC type.

The amount of milliamps required to trip also varies normally by a factor of 3 so 10ma, 30ma, 100ma, and 300ma the latter two often supplied with built in delay (S type) to ensure the 30ma will trip first although not uncommon for a fault to take out all RCD's.

In the home the 30ma is now the norm which must trip within 40ms when supplying buried cables (under 50mm) and unreserved sockets but with TT earthing systems the 100ma was used to compensate for the poor earth now with the rule book stating 300ma is required for fire protection it is likely in the future the 300ma will become more popular where two RCD's are used in series.

In the main today only 30ma RCD's are used in the home although you may find a 10ma feeding specials like a garden pond.

The electronic RCD today is far better at not tripping with a spike but they also need an amount of voltage to work so away from the consumer unit we again have two types the passive as used in the consumer unit will stay in during a power cut the active will automatic disconnect once the voltage drops to a point where the electronics may fail in other words they fail safe.

An RCBO is a RCD and MCB combined and often they monitor line and neutral but only switch the line. They often have a special white lead to insure they will still trip with a neutral fault.

I personally was considering fitting an X-Pole but the RCBO has come on in leaps and bounds and now I would say the RCBO is a better option.

There will always be pros and cons as it is should I lose power to my fridge/freezer due to RCD trip my landing light will light (emergency type with built in battery) so showing me that I have lost power. If I used RCBO's one could trip and I could be unaware. My stand alone freezer has nothing to tell me of a failure but my fridge/freezer has little blue LED lights on the door so likely I would notice if they went out.

But in the main RCBO's trip less than RCD's with no real fault and my mother has RCBO's in kitchen which have never tripped except when I plug in the RCD tester once a year to test them.

As well as the milliamp rating the RCD or RCBO also has two amp ratings. The main one shows how much it can switch on a regular basis and should be bigger than either the sum of MCB's it feeds or supply fuse. The second is how much current it can stand for the few milliseconds it takes for the MCB or fuse to open and must be greater than the prospective short circuit current or let through value of the fuse. Often around 4.7kA or 10kA I have tested and found I can't use 4.7kA version but not found a value over 10kA except when right next to the transformer it is unlikely to be a problem.

I have come across a few new RCD's which failed with the RCD tester I think these were stressed by the feed in and out cables warping the case slightly and causing a failure. However once fitted and tested very few fail latter. However it is rather important that all RCD's are tested when fitted and not with the built in button but with a proper tester which can measure the time taken. 40ms is a little too quick for a stop watch.

From what I can see the cheap one is rated 6kA and the Wylex original rated at 10kA so the question has to be what readings do you get with your loop impedance tester as to the prospective short circuit current? It's all down to your inspection and testing results.

 

[stillp]

Ask yourself why it's half the price of Wylex, themselves a cheap brand.
Also, by mixing brands you'll have no valid guarantee against either CTI or Wylex.

 

[AndyPRK]

Have a look at volex rcbo's. Are they any cheaper than wylex?

 

[PrenticeBoyofDerry]

As Eric has pointed out, the protective devices are type tested to the board.
It is important for safety reasons that compatible devices are installed.

Saving money rarely saves lives!

 

[stillp]

As Eric has pointed out, the protective devices are type tested to the board.

And of course, vice versa!

These copiers have no idea what are the tolerances on the original manufacturers product, they just measure up a sample and copy that, often in the wrong materials.

 

[ericmark]

I have asked a few times with no real answer what stops us using a distribution unit rather than a consumer unit?

However what ever is used or what ever name is given to it at the end of the installation it needs testing and inspecting.

In the main DIY people don't test because simply the equipment is too expensive so as we look at a DIY job we see a whole list of short cuts taken.

1) The prospective short circuit current is unknown.
2) The earth loop impedance is unknown.
3) The insulation resistance is unknown.
4) The RCD tripping current is unknown.
5) The RCD tripping time is unknown.

Even with good quality gear I have found problems often simply cured by slacking off screws and re-tightening which has been highlighted when doing testing.

What you want to do is add to the list by using a RCD which claims only 6kA rather than Wylex 10kA without doing the tests. And use a RCD of low quality without testing.

Much of what is inside the modern RCD is to do with stopping spikes from tripping the device. Even with a proper RCD tester I have found a RCD was tripping on a regular basis but showed nothing as being wrong on the 6 tests normally done but swapped it and all problems went away.

To test a RCD takes 3 tests all done twice once with positive half cycle and once with negative half cycle. You test at 1/2 rated value to show it will not trip and rated value to show it will trip and 5 times rated value to show time it trips in. You can't do these 6 tests with a single press of the test button. The button only tests to see if the mechanical bits have got stuck or not it does not test if electrically sound.

So your going the pay £200 for a RCD tester but want to save £20 on the RCD it just does not make sense.

 

[seasickstevie]

Fully get what you're saying eric, but just to clarify the difference in eight RCBOs is £120 no £20.

 

[JohnW2]

What you want to do is add to the list by using a RCD which claims only 6kA rather than Wylex 10kA without doing the tests.

I've asked this before, but never really got a feel for the answer. Do you electricians (maybe outside of London) ever (or hardly ever?) see PFCs in domestic premises even remotely as high as 6kA, let alone 10kA? Admittedly my experience is very limited, but I don't recall having measured one which was a lot over 1kA.

Kind Regards, John

 

[aesmith]

I personally was considering fitting an X-Pole but the RCBO has come on in leaps and bounds and now I would say the RCBO is a better option.

I saw some discussion where people were concerned that RCBOs only disconnect the live, if I followed their meaning I think the concern was that a neutral-earth fault on one circuit would not be isolated and could still cause unrelated circuits to trip. Is that the case? I suppose the ideal would be two pole RCBOs, which seems an obvious development if they could still be made single width.

 

[JohnW2]

I saw some discussion where people were concerned that RCBOs only disconnect the live, if I followed their meaning I think the concern was that a neutral-earth fault on one circuit would not be isolated and could still cause unrelated circuits to trip. Is that the case? I suppose the ideal would be two pole RCBOs, which seems an obvious development if they could still be made single width.

It's only an issue with a TT installation which has an up-front time-delayed RCD - in which case, as you say, a N-E fault on a circuit protected by a SP RCBO will not be cleared by the RCBO, and therefore will cause the up-front RCD to trip. If all final circuits are protected by RCDs or RCBOs, then there's no need for that up-front RCD, so the problem does not exist but, if there is such an up-front RCD, one should not really use SP RCBOs in TT installations. One can, of course, get DP RCBOs, but they're pretty expensive, and often/usually double-width.

Kind Regards, John

 

[plugwash]

If all final circuits are protected by RCDs or RCBOs, then there's no need for that up-front RCD, so the problem does not exist

If there is no up-front RCD and you use SP RCBOs you have a different problem, there is nothing to disconnect a neutral to earth fault.

 

[JohnW2]

If all final circuits are protected by RCDs or RCBOs, then there's no need for that up-front RCD, so the problem does not exist

If there is no up-front RCD and you use SP RCBOs you have a different problem, there is nothing to disconnect a neutral to earth fault.

Well, although that's true, it's surely no different from the situation of any SP RCBO in any TN installation - and one doesn't hear people expressing concerns about that. The persisting N-E fault won't affect RCDs/RCBOs protecting any other final circuits, and I can't see that the N-E fault, per se, will do any harm (and presumably will be rectified fairly quickly, so that the circuit in question can be re-energised).

... or am I missing something?

Kind Regards, John

 

[ban-all-sheds]

If there is no up-front RCD and you use SP RCBOs you have a different problem, there is nothing to disconnect a neutral to earth fault.

If there's no up-front RCD you don't need to clear EN faults.