RCBOs

[JohnW2]

IEC/TS 60479-1:2005 is the standard describing the effects of AC current, in terms of magnitude and time. Shown here .... We had to be within zone AC-2 ....

Yes, I'm familiar with that, but it leads me to wonder exactly what you meant when you talked about limiting the shock current to 5mA. As your link shows, AC-2 does, indeed, limit the magnitude of shocks to 5mA for durations of 5 secs or more, but with the 100 msec disconnection time you go on to mention, it allows a shock current of something like 80mA.

... and less than 50V touch voltage. We arrived at a touch voltage of 40V.

I'm a bit confused here. One surely designs either for a particular maximum shock current (you indicated 5mA) OR, equivalently, for a particular touch voltage (assuming some value of shock path impedance) - I therefore find your "and" a bit confusing.

The same standard gives a hand to hand impedance of 2600 Ohm at that voltage, and 6500 Ohm hand to foot impedance. That gives approx 6.25mA, and in our case, the OCPD operated in 100mS.

I personally feel that hand-to-hand is a far more important consideration since, unless people have bare feet or are wearing deliberately conductive footwear (including socks) paths to earth via the feet will usually be of sufficiently high impedance to not represent a major risk. Hands are much more likely to be 'naked'. If you were working on the basis of a max touch voltage of 40V and an assumed hand-to-hand impedance of 2,600Ω (which, as I said before, assumes fairly dry hands), that would equate to shock current of about 15mA - still fine for AC-2 with 100msec disconnection, but still very different from the 5mA you said was being aimed for. I still think that hand-to-hand shock path impedances can easily be much lower than that assumed figure, in which case the actual shock current would probably be a lot more than 15mA in some cases (although still less than the ~80mA which would be acceptable for AC-2 with 100msec disconnection time).

Returning to your original point, if you work to a max touch voltage of 40V, that obviously means that (assuming a 230V supply and that both conductors are the same length) that the CPC CSA has to be about 4.75 times greater than the CSA of the L conductor [ (230-40)/40 ] - which is more realistic/do-able than the enormous figure I previously mentioned one would get if one designed for a max shock current of 5mA (the figure you mentioned) and and a body impedance of 1kΩ (which I would personally think would be a reasonable 'bad case' figure).

Kind Regards, John

 

[ericmark]

40 ms seems a long time when one gets a shock, I had a shock at home when I hit a buried cable with a hacksaw blade enough to knock me out, and I have all RCD protected circuits have done for years.
There are two pole switching single module width RCBO's however the neutral is not monitored for over current. They are really for TT installations and would not normally be used for TN. My house the RCD will trip around once a year with no fault, mothers house the RCD has never tripped other than when tested. She has a kitchen with all RCBO's and rest of house on a pair of RCD's.
In the main tripping is caused by some thing plugged in, so having RCBO protection of sockets, maybe one for cooker and one RCD for rest house it is very unlikely that rest of house will ever trip.

 

[ban-all-sheds]

I just looked. A few companies do make DP RCBOs. Not common but readily available. You would probably need a vertical CU, as you see in the USA. Personally I cannot see why RCBOs are not fitted as standard by most sparks. One or two RCDs is just plain naff. Whole banks of circuits can be tripped out which are fault free.

Until the problem occurs of course. Then you have major appliances out because of say a small fault on a lighting circuit. You sound like someone who castigated RCDs when introduced, when many said, "we had no problems so why introduce them?"

You know what? Most safety and security systems will never be used. But when they are, they make a difference. RCBOs are a form of insurance, as are security video cameras.

Faulse economy. Installing all CUs with RCBO is not a great amount of money.

The great thing about having all RCBOs, is that the fault is isolated to only one circuit. The circuits that are fault free stay operational.

With all RCBOs the compromise is vastly reduced. Then with DP RCBOs the compromise is vastly reduced again. RCBOs are not a great expense, for what they offer. It is a no-brainer.

Safety is last on the list after economy.

If your young daughter was electrocuted because someone wanted to save a few quid, you might think differently.

There is a greater chance of being protected via an RCD or RCBO than not having them. Pretty obvious really.

Anyone would think you were being told that you may not have an all-RCBO installation.

 

[JohnW2]

There are two pole switching single module width RCBO's however the neutral is not monitored for over current.

Does one ever monitor neutral for overcurrent - and, if so, why?

It would seem particularly unnecessary in the case of an RCBO. If, somehow, there over-current in the N but not the L, the residual current part of the device would immediately trip, anyway.

Kind Regards, John

 

[ericmark]

Most duel width RCBO's are designed to measure the current in both poles, they can be used for split phase supplies as well as TN supplies. I had not considered how the unbalance would trip it anyway, you are correct not really needed, however most do have it.

I suppose it is like using a triple breaker to control a three phase motor, it only needs two, but will always use three.

 

[JohnW2]

Most duel width RCBO's are designed to measure the current in both poles, they can be used for split phase supplies ...

Fair enough, but with a 'split load' supply, they are monitoring current in 'two Ls', not in L and N.

I still can't think of any reason why one would actually want/need to monitor (true) N current (assuming that L current was being monitored!).

Kind Regards, John

 

[hard-work]

..

Best you understand what I was asking.

 

[ban-all-sheds]

Do you?

 

[hard-work]

..

Please go away. You have far too much time on your hands.

 

[securespark]

What if your young daughter was electrocuted because somebody thought an RCD would protect her?

 

[SimonH2]

Faulse economy. Installing all CUs with RCBO is not a great amount of money.

Especially when there are compact, 2 pole switching, devices available for the cost of many manufacturers' MCBs.
https://www.sbstradesales.co.uk

 

[EFLImpudence]

Well, £15.00 plus vat (£18.00) is still a lot dearer than £2.20.
Extra £126.40 for eight.

https://www.toolstation.com/shop/Electrical/d190/CED+Consumer+Units/sd2979/MCB/p17955

 

[Iggifer]

Especially when there are compact, 2 pole switching, devices available for the cost of many manufacturers' MCBs.
https://www.sbstradesales.co.uk

I would not be fitting that in my own house that is for certain!

 

[flameport]

https://www.sbstradesales.co.uk

Doubtful they sell many or any, as you can only order after applying for an account, part of which includes listing qualifications to supposedly prove you are competent to install such things.
They do not accept credit cards, and the price list implies that if the order is lost by their shonky carrier, it's only covered for £25.

 

[JohnW2]

Doubtful they sell many or any, as you can only order after applying for an account, part of which includes listing qualifications to supposedly prove you are competent to install such things. .... They do not accept credit cards, and the price list implies that if the order is lost by their shonky carrier, it's only covered for £25.

Indeed, and I'm not sure that they would necessarily be regarded as flavour of the month by this particular forum, either ....

We have been involved in the Electrical Industry, both here and in North America since 1958 and have seen many changes to our profession over the years. The most frustrating change has been the emergence of the DIY culture and the sale of technical products to the general public.

Kind Regards, John