Whats safer?

Is a 240v system safer with MCB's and RCBO or a 110v system?

Just trying to get my head around building sites insisting on 110v and i would like to gain knowledge of why when as i understand all CU units of late have MCB's and RCBO's ? (please correct me if wrong here)


On a building site we have yellow leads tracing for miles with all types being dragged over them , getting damaged, items being stored on top of flex, flex being still rolled up etc when we have a 240v socket on the wall we are working in. Not 100 meters away at the 110v transformer with all types of extensions, multiple pugs etc coming out of it.

So what is the deal here? Which do you think is safest overall?

I believe regulation is the order of the day. Portable tools to be run from 110volt centre-tapped transformer such that there is no more than 55 volts to earth.

MCBs on 240 offer no protection - they are there to prevent cable overload.
RCBOs might offer some protection but they can fail so offer some, but unreliable, protection.

110V is miles safer.

RCD's don't prevent or lessen the severity of shocks, they just hopefully cut the power before you die, and being mechanical devices, they can fail.

The 110V supply actually comprises of two 55V lines (compared to earth)
Given the resistance of human skin, you may not receive a shock, or if you do it will be much less severe than the full 230V

The down side of lower voltage means larger cables and restrictions on the length they can be, but for construction power it's not too much of an issue.

Hopefully will get some more replies to add to this.


If we was to way up "accidents" of electric shock v trip hazards etc from running cables then what would your guess be on which is worsed?

For example , if you was to pick up a accident book on a building site, well would you find reported cases of e;ectric shock or genural accidents from tripping etc?

VERY VERY rare to read electric shock from when i was out on site as an assessor, but is this because its 55v to earth that most getting a shock barely feel it let alone report it ? (so our sites are very safe regarding electric)

Is 110v out of a American (is it America thats 110v? ) socket different to our site transformers, you keep saying 55v to earth. Is the American sockets 110v to earth ?

I take it English isn't your first language. Is so, well done with asking your question.

Yes, the American 110 outlet is referenced to earth, rather than centre tapped 55 - 0 - 55, so potentially (excuse the pun) much more hazardous.

mattysupra said:

On a building site we have yellow leads tracing for miles with all types being dragged over them , getting damaged, items being stored on top of flex, flex being still rolled up etc when we have a 240v socket on the wall we are working in. Not 100 meters away at the 110v transformer with all types of extensions, multiple pugs etc coming out of it.

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That's why they make those 3kVA portable site transformers.

Jackrae said:

I take it English isn't your first language. Is so, well done with asking your question.

Yes, the American 110 outlet is referenced to earth, rather than centre tapped 55 - 0 - 55, so potentially (excuse the pun) much more hazardous.

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Sorry, yes English is my first Language, Im Dyslexic so struggle with writing, paragraphs etc .
I left school with poor grades in English and Maths but great with other subjects. Did well after leaving school and went on to represent the U.K 3 years running in the European Championships in my trade, that was after winning British Champion 3x running . I was also the Youngest ever to achieve masters level.
I now personally own Europe's largest independent training centre where i help others who cant spell, write a correct sentence etc to become very good practically skilled. Yep i cant help everyone but i do my best as i was always put down by teachers and others who took the pee out of me when i struggled to spell etc. I cant teach Common sense tho thats for sure so im sorry i cant help you there.

but anyway, thanks for your comment. [/u]

ban-all-sheds said:

mattysupra said:

On a building site we have yellow leads tracing for miles with all types being dragged over them , getting damaged, items being stored on top of flex, flex being still rolled up etc when we have a 240v socket on the wall we are working in. Not 100 meters away at the 110v transformer with all types of extensions, multiple pugs etc coming out of it.

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That's why they make those 3kVA portable site transformers.

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Most sites i worked on they would not let you plug in a yellow brick in the room you was working in. You would plug in near site office or use the sites large transformer to plug in. On the sites you could plug into the socket in the room you was working in then they would let you use 240v.

Seems to be many different rules depending on which site and where the site is in the uk tho.

It depends where the 110 comes from. The 55 - 0 - 55 or 63.5 - 0 - 63.5 supply from a large transformer with twin pole 16A MCB's feeding 16A sockets is much safer than the 0 - 230 volt even when the latter has RCD protection.

The 55 - 0 - 55 from a yellow brick transformer is however likely quite a high fire risk but not because the system is flawed but the regulations are not being followed.

A 3 kW yellow brick is typically protected by a 13A fuse and trip both installed on the input. As a result it needs 54 amp at 55 volt to get 13 A to flow in the primary. With extension lead plugged into extension lead the loop impedance is often too high to allow 54 amp to flow so a short circuit line to earth will not open the protective device. In the end the insulation will melt closer to the transformer and then enough current may flow but often fire results first. So in general the yellow brick transformer is very dangerous.

However as I have said this is because they don't comply with regulations not because there is a fault with the regulations.

As to RCD protection the regulations Note 2 411.3.3 says it's not required for 110 volt systems although you can get them.

411.8.3 states:- The earth fault loop impedance at every point of utilisation, including socket-outlets, shall be such that the disconnection time does not exceed 5 s. This means most the yellow bricks do not comply.

The reduced low voltage is required with "CONSTRUCTION AND DEMOLITION SITE INSTALLATIONS" but no where else. But where tradesmen work in both construction and repair clearly they do not want to duplicate on their tools.

In a domestic for example having a long 230 volt extension lead to a transformer then having the tool plugged directly into the transformer complies with the regulations as the lead to power tool is that short it would trip the supply to the brick. But reverse that and have long extension lead after the transformer it would not likely comply as it would fall foul of 411.8.3.

On a construction and demolition site installation the yellow brick is not required as there are no points where 230 volt at 13A is available only the 16A 110 volt supply which is protected with a double pole MCB is available.

So in theroy the 110 volt reduced low voltage is the safest. In practice it's a risk assessment between shock and fire.

Personally I would ban the yellow brick either a proper reduced low voltage complying with all the rules or nothing. Fire on building sites is a very real problem.

Two other points raised.

I am dyslexic and rely heavily on a spell checker. Dragon naturally speaking does help but I found it's not so much the spelling as grammar which is a problem. In the main when miss spelt people can work out what one says. Only the idiots who seem intent of pointing out ones disability even mention it. But missing out punctuation is not addressed with a spell check and can alter the meaning.

As to USA system it is not 110 volt as we know it. They have a number of systems including the hot wire and held onto the DC supply right to the end of the 20th century. In the main they have 120 - 0 - 120 so cookers are still 240 volt. So although sockets are 120 volt 60 Hz the large appliances are still 240 volt. France uses this with 230 volt often their cookers are 460 volt that's 230 - 0 - 230 there are some supplies in the UK which a split phase as well mainly farms.

The big problem is we do not fuse neutrals. We may use twin pole MCB's but having a fuse on a neutral is dangerous.

This concept of split phase even with electricians can be hard to grasp. This is not helped when you open a 110 volt plug marked L and N when really it should be marked L1 and L2. For industrial electricians who work with L1, L2, and L3 every day having L1 and L2 is easy. But for those electricians who have not experienced three phase to grasp split phase is some times hard.

I have so often heard the term 2 phase which is wrong as there are not two phases they are in phase with each other so still a single phase but split. If electricians can't grasp the concept what chance is there for a layman?

Electrics is all about maths and I have to admit maths is not my strong point. Imaginary numbers play a major role with electrics and as engineers we call it J everyone else it's I. This is required when we go into power factor correction.

Oddly I could grasp the concept of imagery numbers but not calculus and as to statistics I can understand the curve but working it out is beyond me which is why I only got level 5.

So to recap you have 55 - 0 - 55, then 120 - 0 - 120 and the 230 - 0 - 230 volt two are used in UK centre only in USA.

ericmark said:

I have so often heard the term 2 phase which is wrong as there are not two phases they are in phase with each other so still a single phase but split. If electricians can't grasp the concept what chance is there for a layman?

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I don't think the term 2 phase is wrong. They are not in phase with each other, they are 180˚ out of phase.

Most of the big sites I work on won't allow extensions strung all over the place willy nilly. The fixed temporary power is fixed up out the way to the building fabric and temporary temporary power for drills and the like has to be laid in a sensible way to reduce the risk of tripping hazards.

Whilst a lot a lot of people moan about all the "health and safety gone mad" construction sites are actually quite safe places to work if all the rules are followed.

winston1 said:

ericmark said:

I have so often heard the term 2 phase which is wrong as there are not two phases they are in phase with each other so still a single phase but split. If electricians can't grasp the concept what chance is there for a layman?

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I don't think the term 2 phase is wrong. They are not in phase with each other, they are 180˚ out of phase.

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Indeed so. In fact, if they were in-phase, then the pd between them would be zero - hardly useful for powering, say, a 240V load from the two phases of a 120-0-120 supply

Kind Regards, John

RF Lighting said:

Whilst a lot a lot of people moan about all the "health and safety gone mad" construction sites are actually quite safe places to work if all the rules are followed.

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Are they still overseen by a Clerk of Works like the office block site I was on in the late 80's?

Fanatical about following the rules, he went apeshit when I loosened off a pipeclip so I could thread a cable behind it.

I was threatened with permanent exclusion from the site for the heinous crime of doing another tradesman's work. Threatened calling in the unions, the works!

What I should have done was to find the CoW and inform him I needed this work doing, then he would have assigned a plumber to the job....

RF Lighting said:

Whilst a lot a lot of people moan about all the "health and safety gone mad" construction sites are actually quite safe places to work if all the rules are followed.

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Workers replacing the track on a railway on a scorching hot summer day were required to wear orange full hi viz trousers and jackets. There were no trains as there was no track and they were manually moving ballast ready for the new track to be laid..

It is said that two workers required hospital treatment for heat related illness and pallets of bottled water were delivered to site late in the day.