Kitchen appliance service man advise us to upgrade 20A MCB to 32A! (Rant)

[eveares]

We had our Electrolux combi microwave/fan oven replaced with a brand new one under warranty as our old one was written of due to severe rust, what prevented a previous repair man being unable to get to and replace the heater element what was leaking current to earth and tripping the RCD.

I was working, so it was my mum who was dealing with the installation of the replacement.

However the service man who was replacing the microwave fan oven told my mum that if we are getting nuisance trips, then you should upgrade the 20A type B MCB to a 32A type B MCB.

(The tripping in the past was only even the RCD, not the MCB; I guess he could not figure that out.)

The service man surely could see the microwave fan oven was wired in 2.5mm² T&E, which surely he would have known would be unsuitable for a 32A MCB!?

Anyhow, the the 20A MCB feeds a 3.5KW steam oven and the 3Kw microwave fan oven that was replaced. (Separate cable from 20A MCB to each oven, each with there own isolator.)

So 6.5Kw / 28.26 Amps @ 230v in total.

Once you have applied diversity (10 A + 30% balance), I make it as 10A + 5.48A for a total of 15.48 Amps, what is fine for the 20A MCB.

Ok, we do have a socket on the bottom ovens isolator what is not used and tucked away behind a pull out draw, and yes the 5A diversity for that socket would take us over 20A by half a amp.

I will probably relocate the bottom ovens isolator for a dual pole 20A one at some point as it's very hard to reach at the moment.


Anyway, rant over about inappropriate advice from a appliance service tech.

Regards: Elliott.

 

[ericmark]

For my sins I once worked late 80's for a firm fitting cookers, ovens, and hobs, we worked as a pair, gas man and electrician, the gas man had every bit of kit provided, as electrician nothing, previous job anything which was either expensive or needed calibration was provided, so I had never bought my own loop impedance meter, had a insulation tester and RCD's were rare, but I looked at the homes we were working in and the electrics and I was not happy connecting many of the appliances up.

I decided I did not want my name on the work without testing so found another job, lasted about 2 weeks.

However it was an eye opener, seemed around ½ the gas jobs were refused, he got his tape out and measured and if ½" too close to cupboard it was a drawing air through teeth and jobs worth. Parking was a real problem. And the grease and muck behind the cookers was horrid.

I am sure most good electricians would do same as me, and find a better job. OK today firms can't get away with not providing tools, in fact where I now volunteer we are told we must not use our own tools, as insurance will not cover.

My son for a time did domestic, but like me once he worked on industrial no way was he going to return to domestic, one the pay, but also the interest, soon get fed up of fitting sockets, working on a PLC controlled inverter drive lump of machinery makes you use your brain, not simply following BS 7671 as if it was the holly bible, OK things are changing, I was looking at the problems charging electric cars, with a TN-C-S supply, but even then it seems electricians look at BS 7671 to try and make it safe, and not use their own brains.

I think we know with a single phase supply TN-C-S is not really safe without using a reference earth rod. And I ask the question what does a domestic electrician do when he finds out what he has been doing is not now considered safe? We saw the ELCB-v role out, and then the problems, but few electricians went knocking on doors saying you know that work I did 5 years ago, well it is now not considered safe, I need to alter it, and charge you more money.

I think the domestic electrician does have a hard job, but not electrically but dealing with Joe public who do not like spending money. And the old phrase is still true, pay peanuts and you get monkeys.

 

[SUNRAY]

Parking was a real problem. And the grease, condoms, needles, underwear and muck behind the cookers everything in nightclubs is horrid a dam sight worse.

I am sure most good electricians would do same as me, and find a better job. OK today firms can't get away with not providing tools, in fact where I now volunteer we are told we must not use our own tools, as insurance will not cover.

My son for a time did domestic, but like me once he worked on industrial no way was he going to return to domestic, one the pay, but also the interest, soon get fed up of fitting sockets, working on a PLC controlled inverter drive lump of machinery makes you use your brain, not simply following BS 7671 as if it was the holly bible, OK things are changing, I was looking at the problems charging electric cars, with a TN-C-S supply, but even then it seems electricians look at BS 7671 to try and make it safe, and not use their own brains.

I think we know with a single phase supply TN-C-S is not really safe without using a reference earth rod. And I ask the question what does a domestic electrician do when he finds out what he has been doing is not now considered safe? We saw the ELCB-v role out, and then the problems, but few electricians went knocking on doors saying you know that work I did 5 years ago, well it is now not considered safe, I need to alter it, and charge you more money.

I think the domestic electrician does have a hard job, but not electrically but dealing with Joe public who do not like spending money. And the old phrase is still true, pay peanuts and you get monkeys.

Apart from a spell in the 70's where I slid into wiring new builds for a friend my paid work has been basically commercial and I really hate the hassle and crap that goes with domestic, firstly dealing with the customer and their detritus and secondly trying to work out what botches have been done by the succession of undertrained/inexperienced pro's and DIYers. I'm not putting all house bashers in that category but one only has to see some of the boasting on YT these days and so many times I just think 'really' or 'WTF'. If that's the 5h1t they're boasting about, I hate to think what some of their other work is like.

There is a great deal of satisfaction opening a big control panel with no documentation and working in it for hours, days and sometimes weeks fault finding/modifying/updating and at the end closing the doors on another good job and in the meantime the electricians doing the remainder of the site work saying 'how do you know where everything goes/how it works/what's what?' etc.
These days I've basically retired but still get asked to go to 'just jobs' or after several others have failed to sort things out and unfortunately most seems to be domestic switches and ceiling roses these days.

 

[BS3036]

I think we know with a single phase supply TN-C-S is not really safe without using a reference earth rod.

I would appreciate a reference in BS7671 for this.

 

[JohnW2]

I would appreciate a reference in BS7671 for this.

That was obviously only eric's personal opinion, but we need to remember that it was only at the very 11th hour that this (with my emboldening) was removed from the draft of BS7671:2018 (so we may well see it re-appearing in "the 19th", or even in an Amendment to 18th) ...

542.1.201 The main earthing terminal shall be connected with Earth by one of the methods described in Regulations 542.1.2.1 to 3, as appropriate to the type of system of which the installation is to form a part and in compliance with Regulations 542.1.3.1 and 542.1.3.2. Additionally, there shall be an earth electrode, supplementing any earthing facility provided by the distributor, in accordance with one of the requirements of Regulation 542.2.3, to prevent the appearance of a dangerous touch voltage in the event of the loss of the main connection to Earth.

Kind Regards, John

 

[bernardgreen]

I think we know with a single phase supply TN-C-S is not really safe without using a reference earth rod.

I would appreciate a reference in BS7671 for this.

There probably isn't a reference in BS7671 or any other regulation. That said "exporting" the "Earth" from a house to an outbuilding is mentioned in some regulations as being a matter for concern.

The "unsafe" situation is when a cable fault in the local distribution network breaks the Neutral between the sub-station Earth electrode and the cut out in one or more houses. The Neutral conductor into the house now rises to some voltage above Ground and hence the CPC ( Earth wire ) in the house is no longer at Earth / Ground potential.

In the house this is unlikely to cause any hazards to people unless the body of someone in the house is making a path between an "Earthed" object inside the house and something outside the house that is Grounded. Also some using an Earthed electrical appliance in the garden would be at serious risk of electrical shock.

An example is a gas meter reader on damp Ground wiping clean the window on the gas meter. Due to the ( safety ) bonding of the gas meter to the CPC the gas meter is not at Ground potential but the meter reader's body is at Ground potential. ( One meter reader I knew always wore gloves after getting a mild shock from a gas meter ).

Another risk exists when a bonded water supply pipe is connected to a metal water main in the street. The current that would normally flow along the network Neutral back to the sub station instead flows along the bond to the water mains. This can be a very high current, hence the requirement that bonding cables have a large cross sectional area.

 

[EFLImpudence]

There probably isn't a reference in BS7671 or any other regulation. That said "exporting" the "Earth" from a house to an outbuilding is mentioned in some regulations as being a matter for concern.

Why any more of a matter for concern than anything or anywhere else?

Assuming the supply enters the front of a house, is it considered "exporting the Earth" to a rear room or a large extension which might be the same distance away as an outbuilding?

 

[JohnW2]

Why any more of a matter for concern than anything or anywhere else? Assuming the supply enters the front of a house, is it considered "exporting the Earth" to a rear room or a large extension which might be the same distance away as an outbuilding?

I think the concerns are about "exporting an 'Earth' (which may sometimes be well above true earth potential)" to somewhere where (or nearby where) there may be things at true earth potential (e.g. the actual 'earth' surrounding an outbuilding), aren't they?

As often discussed, outside taps can be as much as a 'cause for concern' as outbuildings.

Kind Regards, John

 

[bernardgreen]

Assuming the supply enters the front of a house, is it considered "exporting the Earth" to a rear room or a large extension which might be the same distance away as an outbuilding?

The rear room or a large extension are still within the equipotential volume of the building and it's CPC. If however one of them had a bare earth floor then a failed Neutral would create a risk of electric shock to anyone on the bare earth floor and holding touching anything that was conductive and connected to the CPC

 

[EFLImpudence]

I think the concerns are about "exporting an 'Earth' (which may sometimes be well above true earth potential)" to somewhere where (or nearby where) there may be things at true earth potential (e.g. the actual 'earth' surrounding an outbuilding), aren't they?

Does that not apply to the rear of a building?

As often discussed, outside taps can be as much as a 'cause for concern' as outbuildings.

Exactly.

Who has TT'd a tap?
Fit a bit of plastic pipe if concerned.

 

[EFLImpudence]

The rear room or a large extension are still within the equipotential volume of the building and it's CPC.

Then call it "extending" the equipotential 'volume'; not "exporting".

 

[bernardgreen]

Then call it "extending" the equipotential 'volume'; not "exporting".

The equipotential volume is determined by the building and is ( normally ) all parts of the building where it is impossible to come into contact with anything connected to the ground outside the building. Metallic pipes coming into the equipotential volume are bonded to the CPC to prevent them in-porting into the equipotential volume the effects of their connection to the ground .

 

[EFLImpudence]

Is it?

 

[BS3036]

I would simply make these points:

• The change in the earth electrode rule was removed. I suppose it was decided it was an over-reaction.
• BS7671 is a pick and mix of international regulations. This probably arose from one of the many countries who standardised on TT supplies.
• As a point of interest, a Danish colleague of mine is convinced that TT will be adopted internationally.
• Making the statement that we are all in imminent danger from TN-C-S seems to be to be rather hyperbolic.

 

[bernardgreen]

Making the statement that we are all in imminent danger from TN-C-S seems to be to be rather hyperbolic.

There was no such statement. The postings point out there is a risk of harm when certain things happen coincidently and it is better to be aware of these things.