110volt transformers with centre tap 55volts???? confused?

[RF Lighting]

I might be missing something here as this is getting a little in depth for me, but I fail to see the danger in your scenario.

I understand that an RCD wuold not trip, but the earth doesn't get above safe touch voltage, so there is still no hazard.

 

[NotHimAgain]

Eric
This will all get too complex, and most will have no interest in it.

First the 110 volts system is well established in the UK and has been shown to be inherently safe if we have faith in the ideas of Biegelmeier and Lee (the touch voltage concept).

The calculation of the touch voltage was carried out because one of our EU buddies tried to claim that the system was not safe. The basis for this was that it has a system voltage to earth is > 50 volts and this is the touch voltage limit for an indefinite disconnection time.

The system voltage to earth is not the touch voltage, so we (the BS 7375 committee) decided to respond.

The calculation was carried out for the BS 7375 committee in the early 90's. I did the actual work, but the results belong to the committee.

The diagram you have drawn has a number of errors, including:
1) the connection to the supply earth from the centre tap of the transformer is solid and will be of low impedance - not 2000 ohms as you have shown;
2) you have not shown the cpc to the appliance, nor have you shown a zero impedance fault at the appliance.

The calculation considered three types of fault placed both near and far from the transformer.

1) a simultaneous short of both lines and the cpc (spade through a cable ).
2) a short between cpc and one line - load intact.
3) a short between cpc and one line - load removed.

The analysis took account of the complex impedances.

The transformer impedances (primary circuit and secondary circuit) were considered, but it was found that they could be eliminated from the touch voltage calculation.

The main (worst case) results (touch voltage - approximate values) are as follows

Fault type----------Single phase----------Three phase
1-------------------- 0 volts-----------------37 volts

2--------------------28 volts----------------32 volts

3--------------------28 volts----------------32 volts

Note that an RCD would not detect fault type 1 for single phase provided you hit all three conductors at the same time .

Superposition Theorem from Wikipedia

The superposition theorem for electrical circuits states that the total current in any branch of a bilateral linear circuit equals the algebraic sum of the currents produced by each source acting separately throughout the circuit.

To ascertain the contribution of each individual source, all of the other sources first must be "killed" (set to zero) by:

1. replacing all other voltage sources with a short circuit (thereby eliminating difference of potential. i.e. V=0)
2. replacing all other current sources with an open circuit (thereby eliminating current. i.e. I=0)

This procedure is followed for each source in turn, then the resultant currents are added to determine the true operation of the circuit. The resultant circuit operation is the superposition of the various voltage and current sources.

The superposition theorem is very important in circuit analysis. It is used in converting any circuit into its Norton equivalent or Thevenin equivalent.

So we take out each half of the secondary winding in turn, calculate and then superimpose it all back together - easy really .

 

[ericmark]

It was not 2000 ohms it was 200 last O was the O of Ohms but does not really matter seems the superposition theorem is what I did in Uni under Norton or Thevenin and I remember what a *** it was so I am not going to try working it all out.
It is interesting to see your results. I realise my lash up was just that and one would have to be very unlucky to get a shock and even then unlikely to go permanent damage.

However I still question the idea of using 110 volt portable transformers where 13A outlets to BS7671:2008 are available.

The first time I found a melted 110 volt lead it was blamed on welders not earthing their welding at point of weld.

As I found more and more melted 110 volt cables I and others started to get worried.

So each time it happened we would try to find out why. Normally we just found the discarded cable but then we realised all in one area.

Then we found it. There was a transformer protected with 12 amp push to reset MCB on input and there was a faulty mag mount drill with intermittent short line to earth on the end of two long extension leads.

This would allow something like 50 amps to flow without tripping MCB. And with 1.5mm flex it was melting.

We found quite a few of these transformers on site and quick fix was to use 4 in-line fuse holders with 13A fuses to the two 16A outlets.

But most were hired and it was an impossible task to keep track of what came on and off site.

As time has gone on again and again I have come across problems with 110 volt supplies from US imports with 110-0-110 volt to RCD's opening when generators dropped to idle.

Also where control transformers have been changed from 0-110 volt to 55-0-55 volt and as a result one leg was left unfused not only that unswitched as well. That caused a very expensive burn out.

110 volt if used correctly is a good idea but sorry to say many including electricians don't seem to understand what they have. The continued practice of using brown and blue instead of brown and black as the line wire colours does not help and many fail to switch or fuse the second line feed.

The whole idea of no neutral on a single phase supply seems to be hard for many to grasp. For those who work on three phase it's not so hard to grasp but for people who spend most of their life wiring houses is seems to be something they can't grasp.

The fire risk is high and in spite of sites banning any hand lamps over 60W the fires still happen. Maybe less caused by halogen lamps but the whole idea of flexible cables being viewable throughout there length seems to be forgotten. 413.3.4

I have seen RCD like this

used without too much problem except for dropping out when used with generators so it has been shown they can be made to work in rough conditions and I am sure they could be incorporated within the transformer case. Maybe it will add £39.14 to cost of transformer but that is less than has been added to cost of a consumer unit so what's the problem.

But how can one argue for something to be added because other things maybe wrong?

Same was as caravans are required to have a 30ma RCD fitted in spite of all supplies to caravan also requiring a 30ma RCD and one does not have to be brain of Britain to realise the only time a RCD is required in a caravan is when the site owner has broken the rules.

Of course in the transformer you do not really require a 30ma RCD as it is in the main for fire risk and a 300ma RCD would do the trick. However as yet the regulations do not seem to have allowed for that so we are still stuck with 30ma. Or are we?

 

[Pensdown]

This will all get too complex, and most will have no interest in it.

If work ever gets tight there’s no shame in perusing a career in mind reading which you obviously have a flair for.

 

[curiouser]

sometime back[but still on this site] eric and co had a very interesting theoretical discussion about 110 transformers BUT in practice on site--
1 can you fit an rcd on the outgoing line from the 110v tx to the site tool
2 what protection would it provide
3 if 240v rcds are sold for domestic in line use why would a 110v one not be equally robust[as implied in the discussion]
4 is there any extra safety to be gained by putting a 240v rcd on the incoming line to the 110v tx [eg if operative falls into a puddle carrying the live tx]
5 does anyone in the uk supply 110v rcds or are these to be imported from usa [110v bing standard]
6 in considering these Qs. what is USA site practice or even domestic USA practice with lawnmowers etc
thanks in anticipation

 

[ban-all-sheds]

5:

To name but one: http://www.mkelectric.com/en-gb/Pro...andoModularCombi/RCD110vdp/Pages/default.aspx

 

[Monkeh]

4 is there any extra safety to be gained by putting a 240v rcd on the incoming line to the 110v tx [eg if operative falls into a puddle carrying the live tx]

Not really, seeing as you're not meant to take 240V to the tool location and then use a transformer. It defeats the purpose.

 

[Spark123]

A 240v RCD connected to the primary will only protect the primary windings, it will not provide any additional protection to the supply from the secondary at all.
The build of a safety transformer should ensure no electrical connection between primary and secondary windings, just a magnetic coupling so a fault between the two windings causing 240v to be fed onto the secondary is a negliable scenario.
Dropping the TX in water shouldn't happen as Monkeh says.
All 240v sockets should be RCD protected anyway.
A 240v RCD is sort of a nonsense anyway as an RCD works on current unless you're talking about RCDs with electronics in them. The test circuit resistor in a 240v RCD is sized for 240v though, that is about the only difference I'd imagine in a bog standard RCD.

 

[RF Lighting]

6 in considering these Qs. what is USA site practice or even domestic USA practice with lawnmowers etc
thanks in anticipation

That's completely different. Our site supplies are 55V to earth, where as US supplies are 110V to earth.

 

[ericmark]

Forgetting about the rules I think 55 volt is unlikely except in very wet conditions to give a fatal shock.

The problem with UK 110 volt is fire. Many transformers only fuse the incomer which if we consider a 16A output would need to be over 6A so next step is 10A. But with a short to earth only one side is grounded as the 2300W input will give nearly 42A at 55 volts.

Now the normal 1.5mm flex does not like 42A and will likely melt and set things around it on fire.

So in real terms it is the fire risk we need to protect against. Be this fuses on output or RCD on output does not really matter. The high impedance of the transformers does not help.

As with many things it is a compromise. Lower the voltage less problems with receiving shocks and higher chance of fire.

Although the 55-0-55 or 63-0-63 volt supply from a large transformer does increase site safety. The supply from the Yellow brick transformers does not really add to site safety. Their job as far as I see it is to allow us to use 110 volt equipment from a 230 volt supply so we do not need two sets of tools.

The fire risk with Yellow brick transformers to my mine out weighs any shock risk when we consider all sockets under 20A not have RCD protection but supplies from the Yellow brick have very little protection.

There are in line 110v RCD's on the market but why use 110 when the 230 volt supply is RCD protected?

However I think this post is a few years old and really one should start a new post and not re-start this old one!

 

[curiouser]

thanks to all who replied including eric who kicked off the discussion.
i seem to have misled you all--

1 240v incomer has its own rcd on shortish lines to portable transformers

2 from tx to site tools lines are 2.5 artic

3 Q is there any safety merit in fitting 110v rcd [other articles say 240 v rcds are not suitable for 110v] in outgoing line from transformer on a belt and braces approach to operative safety-- [use is agricultural/horticultural so fire outdoors not a problem i hope] as operatives will be in contiuously "wet" conditions

4 ref to usa 110v was to see if they use 110v rcds at all as 110v will be their onsite voltage for power tools as well as normal domestic use where uk recommends 240v rcds for lawnmowers,saws etc. do us authorities give similar advice to their 110v citizens-- if they do they must think it adds to safety so uk employers ought to offer best protection available at least as good as us

5 if they make 110v rcds do we need to import them from usa or does any uk co make them

 

[bernardgreen]

I might be wrong on this.

An RCD is current operated, they monitor the difference in the currents flowing on the live and neutral conductors through the unit. The sensor and trip mechanism make no electrical contact to the conductors so voltage is not an issue in the sensor operation. ( there is contact for the test resistor and button but these are not part of the sensor and trip mechanism ).

Voltage between the conductors requires adequate insulation. Running a 240 volt RCD on 110 volt will not stress the 240 volt insulation.

The gap between the open ( tripped ) contacts must be wide enough that the 240 volts cannot jump the gap in a continuous arc. The gap for 240 volts will be double that needed ( minimum) for 110volts

So I believe a 240 volt RCD can be safely and effectively used to protect a 110 volt supply from a transformer provided there is an adequate earthing of the centre tap ( max voltage to earth 55 volts AC ) or one side of the transformers output ( 110 volts to earth possible )

 

[curiouser]

thanks for reply. do i take absence of replies to mean no-one knows the answers and no-one has any usa experience. surely somebody cares if they are in a ditch and they cut through the110 volt cable on their handtool and live ends catch them?

 

[ericmark]

I might be wrong on this.

An RCD is current operated, they monitor the difference in the currents flowing on the live and neutral conductors through the unit. The sensor and trip mechanism make no electrical contact to the conductors so voltage is not an issue in the sensor operation. ( there is contact for the test resistor and button but these are not part of the sensor and trip mechanism ).

Voltage between the conductors requires adequate insulation. Running a 240 volt RCD on 110 volt will not stress the 240 volt insulation.

The gap between the open ( tripped ) contacts must be wide enough that the 240 volts cannot jump the gap in a continuous arc. The gap for 240 volts will be double that needed ( minimum) for 110volts

So I believe a 240 volt RCD can be safely and effectively used to protect a 110 volt supply from a transformer provided there is an adequate earthing of the centre tap ( max voltage to earth 55 volts AC ) or one side of the transformers output ( 110 volts to earth possible )

The whole idea of active RCD's is when the voltage drops below the point where the electronics will stop working then it will open circuit and fail safe.

However losing power can also be a problem with mag mount drills and like and using an active RCD on 110 volt is likely to cause power failures as voltage dips.

Passive RCD's must be within their voltage range and with 110 equipment only RCD's designed for 55-0-55 volt should be used and used in the transformer so there is very little chance of failure due to volt drop.

Although the BS7671:2008 says RCD's should be used with 110 volt in fact it says where other methods are not used even a 12 volt supply should have a RCD I view this as an error and remembering there work is mainly to reduce fire risk then with 2.5mm attic flex I would not bother with RCD protection myself. I would though look at fuse and MCB sizes and where installed.

USA does use RCD protection but it is called a different name "commonly known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI)" we also use other names, residual current circuit breaker (RCCB), an Earth leakage circuit breaker or (ELCB) the latter comes in two types current and voltage the voltage type have in the main been outlawed so we should call it ELCB-c to show current type.

Because of fire I would like to ban the yellow brick transformers, It would be so easy to fit two in-line fuses inside there is really no excuse. The other problem is welding sets they do melt leads and there does need to be a testing procedure laid down to ensure damaged leads are taken out of service. To show compliance every yellow lead needs a plant number and label saying when it needs re-testing. But on sites with 100's of leads I guess this will not happen.

 

[bernardgreen]

The whole idea of active RCD's is when the voltage drops below the point where the electronics will stop working then it will open circuit and fail safe.

Passive RCD's must be within their voltage range and with 110 equipment only RCD's designed for 55-0-55 volt should be used and used in the transformer so there is very little chance of failure due to volt drop.

As the passives only use the energy from the current sensing transformer to opertate the trip mechansim then they are not concerned about the voltage.

There are higher currents ( for the same load ) with 110 volts but these higher currents cannot saturate the core as they are inopposition in transformer's two primary windings and the effective primary current is zero until an earth leakage occurs.

I have used a 230 volt 30m RCD on a 12 - 0 - 12 volt AC system and it tripped when there was an un-balance. ( this was a "quick fit" during trials of a new system, later it was replaced by a custom built sensor )