Isolation transformer - Generator N-E bonding

[WotEver]

As this has become a bit confusing...

Doesn't it always, once a certain poster gets involved?

 

...an Isolation transformer will allow the output of the transformer to have N&E bonded, and is advised/best practice to bond them.

It's actually essential to bond them if you want the RCD to work.

[nicknorman]

Lets get something straight we do not disagree that if the is an imbalance of 30mA that a 30mA RCD will trip.

 

On a 230V Neutral Earth bonded systems there is 230V between live and earth as well as live and neutral. For there to be a 30mA leakage to earth that would mean there would be an earth leakage amounting to a breakdown of the insulation the equivalent to the insulation failing to a value 7666 Ohms or less.

 

On a centre tapped earth system while there is 230V between live and live there is only 115V between each of the two lives and earth. This means that for a earth leakage amounting to the equivalent of 7666 Ohms insulation failure, the current flow would be only 15mA The breakdown of the insulation failure has to be effectively twice as bad on a centre tapped earth system to reach 30mA or effectively insulation reduce to effectively 3833 Ohms.

 

That means, for say an immersion heater the damage would have to be twice as bad for it to guarantee tripping the RCD as its insulation failed and it leaked to earth on a centre tapped earth system.

 

The general specification for RCDs is that it should not operate at 50% of the specified current. So a 30mA RCD used on a centre tapped earth system should not operate when there is a fault that on a Neutral Earth bonded system that would operate the RCD. See https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwi4uqKL9aHQAhUBfRoKHYJ5DD0QFgg4MAE&url=http%3A%2F%2Felectrical.theiet.org%2Fwiring-matters%2F15%2Finsp-test-rcd.cfm%3Ftype%3Dpdf&usg=AFQjCNH11wg1-o9LooMipkQaakj899MVFQ&bvm=bv.138493631,d.d2s page 8.

 

I think you will find this is why the US uses Earth Leakage Circuit Breakers (ELCB) on their 120V systems which voltage is referenced. Where 240V 2 or 3 phase is supplied for heavy current devices, washing machines cookers etc, there is still only 120V to earth.

 

Ed to correct US voltages

 

 

 

 

Where did I say an IT was best practice?

You said all that back in December 2015, why do you think that the elapsing of 11 months is going to change anything?

 

An RCD is a leakage current detector, not a leakage resistance detector - for the simple reason that it is current that is potentially fatal, not resistance. With a centre tapped system the chances of nuisance trips is reduced, whilst ensuring an identical level of safety. This is why the ISO is happy to allow unpolarised supplies whilst making no change to the standard of RCD installed (other than the need to be double pole).

Edited November 12, 2016 by nicknorman

[Chewbacka]

.............................It's actually essential to bond them if you want the RCD to work.

Agreed & thanks - I should have made that clear.

[Iain_S]

(snip)

I'll also point out that your last two points were both made hours ago before graham geo started confusing matters by saying that an IT was best practice and that a standard RCD wouldn't work.

 

I know, but geo.m seemed to be arguing from a different starting point than everyone else

[WotEver]

 

I know, but geo.m seemed to be arguing from a different starting point than everyone else

Well, yes. I guess I should be used to it by now.

[Tony Brooks]

This is sort of off topic but very relevant.

 

Where the hell are the mods? We have two very knowledgable members trying to give the same advice and one member who seems to be trying to confuse the issue. Dan - take note. You very nearly drove both posters who are, in my view, giving good advice away. Do you want to be running a site that promotes bad advice?

 

 

PS to the OP - although it is outside my strict area of competence I tend to side wit WotEver and Nicknorman.

Edited November 12, 2016 by Tony Brooks

[Geo]

Sorry not to have responded but have been away.

 

I find it interesting how blinkered some people can become. There have been implications that resistance and voltage do not matter as far as electrocution or an electric shock is concerned only current. Well it is wrong otherwise a battery with a voltage of 1.5 volts would be able to electrocute/shock us, it can’t unless our body resistance is very low.

 

In order for there to be current of sufficient level to hurt us, there has to be sufficient voltage to push the current through the body and the body resistance has to be low enough to allow the current to flow through our bodies. So Ohms Law come into effect V/RA.

 

The human body with an intact skin has a resistance which can vary from when wet, 1000 Ohms to with a dry skin 20,000 Ohms and higher. It is very difficult to measure skin resistance because the way the skin reacts to electricity and protects the body’s internal organs is dependent on the voltage being applied to it and the frequency. If the skin is broken at the entry and exit points the resistance drops dramatically and can be as low as 10 or 20 Ohms. So for the sake of average figures I will use a body resistance of 5000 Ohms.

 

12 volts will give a current flow of 2.4mA

48 “ “ “ “ “ “ “ 9.6mA

115 “ “ “ “ “ “ “ 23mA

230 “ “ “ “ “ “ “ 46mA

 

What effects will that have on our bodies?

 

Effects of Electrical Current* on the Body

________________________________________________________________

Current Reaction

 

1 milliamp

Just a faint tingle.

 

5 milliamps

Slight shock felt. Disturbing, but not painful. Most people can “let go.” However, strong involuntary movements can cause injuries.

 

6-25 milliamps (women) †9-30 milliamps (men)

Painful shock. Muscular control is lost. This is the range where “freezing currents” start. It may not be possible to “let go.”

 

50-150 milliamps

Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Death is possible.

 

1,000-4,300 milliamps (1-4.3 amps)

Ventricular fibrillation (heart pumping action not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely.

 

10,000 milliamps (10 amps)

Cardiac arrest and severe burns occur. Death is probable.

_________________________________________________________________

†Differences in muscle and fat content affect the severity of shock.

 

 

For currents above 10 milliamps, muscular contractions can be so strong that the victim cannot let go of the wire that is shocking them. At values as low as 20 milliamps, breathing becomes laboured, finally ceasing completely even at values below 75 milliamps.

 

So what happens when we have a 230V supply with a centre tapped earth with a 30mA RCD is that the current flow for the same fault or body is half of what it would be with a 230V supply neutral earthed system

 

So our body gets:

230V centre tapped earth; 115V to earth 23mA RCD does not trip under 30mA

230V neutral bonded earth, 230V to earth 46mA RCD trips over 30mA

 

RCD does not trip the person gets painful shock. Muscular control can be/is lost. This is the range where “freezing currents” start. It may not be possible to “let go.”

 

So for the same fault that would trip the RCD on a 230V neutral earthed system the RCD on a centre tapped earthed 230V system the RCD is below its trip level and thus will not trip.

 

That is why I do not like 230V centre tapped earthed system on boats in particular unless it is fitted with an a 15mA RCD or a voltage sensing ELCB.

 

[WotEver]

a lot of boring drivel.

 

... none of which is relevant.

[nicknorman]

All very interesting but irrelevant in that the same level of safety is achieved with a standard 30mA RCD on a centre tapped or polarised system. It is true that one could probably get an increased level of safety with a centre tapped system and 15mA RCD for the same degree of nuisance tripping. but there can be no claim that this is required since we are all quite happy with the level of safety procided by a 30mA RCD on a polarised supply.

[Geo]

All very interesting but irrelevant in that the same level of safety is achieved with a standard 30mA RCD on a centre tapped or polarised system. It is true that one could probably get an increased level of safety with a centre tapped system and 15mA RCD for the same degree of nuisance tripping. but there can be no claim that this is required since we are all quite happy with the level of safety procided by a 30mA RCD on a polarised supply.

 

I do not agree as a boater could get, with a fault that on a neutral earth 230V system that will trip the RCD; on a centre-tapped earth 230V system with that same level of fault that will give them a bad shock and could be severe enough to cause injury because the RCD at 30mA will not trip.

 

 

From experience because the voltage on a centre-tapped earthed system is 115V to earth the level of nuisance tripping using a 15mA for the centre-tapped system is no more than with a 30mA RCD on a 230V neutral-earth system with 230V to earth.

[WotEver]

Cleverer folk than you and me have determined that 30mA is a suitable current at which to disconnect the supply in order to avoid serious injury. There are no caveats to that recommendation, however much you choose to squirm and argue.

 

edited for tryping

Edited November 14, 2016 by WotEver

[Geo]

 

... none of which is relevant.

 

lol

Cleverer folk than you and me have determined that 30mA is a suitable current at which to disconnect the supply in order to avoid serious injury. There are no caveats to that recommendation, however much you choose to squirm and argue.

 

edited for tryping

 

Who are these folk?

 

Of course you have criticised the BSS folk and being, what was the word, think it was stupid.

Edited November 14, 2016 by Geo

[WotEver]

Who are these folk?

 

I am amazed that as a self-proclaimed expert in these matters that you are ignorant of the answer to your question. They are the multi-national body who drafted IEC60947, much of which was based on the research of Gottfried Biegelmeyer (who, incidentally, invented the RCD).

BS7671 too, if we wish to talk about British specs.

 

 

Edited to correct the spelling of poor old self-electrocuted Gottfried

Edited again to correct dyslexic 60497 to 60947

Edited November 14, 2016 by WotEver

[Geo]

All very interesting but irrelevant in that the same level of safety is achieved with a standard 30mA RCD on a centre tapped or polarised system. It is true that one could probably get an increased level of safety with a centre tapped system and 15mA RCD for the same degree of nuisance tripping. but there can be no claim that this is required since we are all quite happy with the level of safety procided by a 30mA RCD on a polarised supply.

 

What you are saying is the same level of safety is achieved on both a neutral-earth system and a centre-tapped earthed system using a 30mA RCD. That would mean at 30mA the RCD would protect the average body from shock, yet it does not on a centre-tapped 230V system.

[WotEver]

 

What you are saying is the same level of safety is achieved on both a neutral-earth system and a centre-tapped earthed system using a 30mA RCD. That would mean at 30mA the RCD would protect the average body from shock, yet it does not on a centre-tapped 230V system.

What absolute rubbish! Whether centre tapped or not, a 30mA RCD would trip with <30mA current. It's current that kills, not voltage (or resistance).

[nicknorman]

What you are saying is the same level of safety is achieved on both a neutral-earth system and a centre-tapped earthed system using a 30mA RCD. That would mean at 30mA the RCD would protect the average body from shock, yet it does not on a centre-tapped 230V system.

 

You are incorrect, and this is the nub of the matter: A standard 30mA RCD will trip at 30mA. It matters not one jot whether it is as part of a polarised or a centre tapped system.

[Geo]

 

I am amazed that as a self-proclaimed expert in these matters that you are ignorant of the answer to your question. They are the multi-national body who drafted IEC60497, much of which was based on the research of Gottfied Biegelmeyer (who, incidentally, invented the RCD).

BS7671 too, if we wish to talk about British specs.

 

Could it be that you mean IEC 60898-1

[WotEver]

 

Could it be that you mean IEC 60898-1

 

Insofar as the same body wrote that document also, yes they are the same people.

[Geo]

You are incorrect, and this is the nub of the matter: A standard 30mA RCD will trip at 30mA. It matters not one jot whether it is as part of a polarised or a centre tapped system.

 

So it does not matter one jot that some can touch a live & earth on a neutral-earth system and the 30mA RCD will trip; yet if they touch the live & earth on a centre-tapped system the RCD will not trip, just give them a nasty shock that could injure them. That is just when the RCD is supposed to trip.

Edited November 14, 2016 by Geo

[Geo]

 

Insofar as the same body wrote that document also, yes they are the same people.

 

Ah

Standard: IEC 60497 INTERNATIONAL CODE FOR MODEL ACCEPTANCE TESTS OF STORAGE PUMPS

 

Not sure how that applies to RCDs on boats or domestic premises

[WotEver]

If the imbalance current is anywhere >15mA and <30mA the RCD will trip.

 

 

Ah

Standard: IEC 60497 INTERNATIONAL CODE FOR MODEL ACCEPTANCE TESTS OF STORAGE PUMPS

 

Not sure how that applies to RCDs on boats or domestic premises

 

Sorry, dyslexia - 60947

Edited November 14, 2016 by WotEver

[nicknorman]

So it does not matter one jot that some can touch a live & earth on a neutral-earth system and the 30mA RCD will trip; yet if they touch the live & earth on a centre-tapped system the RCD will not trip, just give the a nasty shock that could injure them. That is just when the RCD is supposed to trip.

 

It will depend on the nature (resistance) of their connection with L and E. In both cases the RCD will trip with current at 30mA. Obviously one can envisage high resistance connections that will give a nasty but non-fatal shock without tripping the RCD in both cases, and one can envisage lower resistance connections that will trip in both cases. Even when the RCD does trip I imagine that the shock will still be of sufficient duration to make one jump a bit! The RCD is not there to prevent one receiving any sort of shock, it is there to prevent a shock of sufficient intensity to be fatal.

[WotEver]

And poor old Gottfried determined much of this by dropping hairdryers into the bath with him sat in it.

Interesting post I came across on the IET forum recently. Just found it again...

 

 

There are two lines of combined protection offered by RCDs and that is:

1/ Disconnection time
2/ Maximum current flow

1/ Disconnection time
This relates to the physiological effects of current on the heart over a period of time.
If the standard heart beat is between 60 -120 beats per minute that would give us a heart cycle of 1 - 0.5s
There is a vulnerable part, presumably where electrical impulses initiate the contractions and hence blood is pumped through the heart of between 10 - 20% of the heart cycle which relates simply to between 50 - 100ms. This would be why the maximum cut-off time for a RCD to offer basic protection is <40ms

2/ Maximum current flow
from 1mA perception of current, to 20 - 30mA where 95% of persons in contact with this level will survive, to 30 - 50mA where the % of survival significantly reduces. So 30mA is taken as the maximum threshold

The two of these factors combine together to give us a set of probabilities of the chance of fibrillation incresing with current flow beyond 1 heart cycle and fatal if the level of current flow, controlled by the body reistance, goes beyond 30 -50mA.

Therefore, for additional basic protection against the physiological effects caused by electric shock 30mA RCDs are used and the time is limited to a maximum of 40ms significantly reducing the likelihood of heart fibrillation

 

You will note that voltage is not mentioned anywhere in the text.

Edited November 14, 2016 by WotEver

[Guest]

And poor old Gottfried determined much of this by dropping hairdryers into the bath with him sat in it.

Reminds me of the German electrician when my daughter moved into her flat. There is a 240V outlet quite close to the bath. She told him that such a socket was not allowed n the bathroom in the UK (at the time) to which he asked "why?" She said in case somebody started using their hairdryer whilst sat in the bath. He said if they are that f*****g stupid they deserve to die!

[Geo]

It will depend on the nature (resistance) of their connection with L and E. In both cases the RCD will trip with current at 30mA. Obviously one can envisage high resistance connections that will give a nasty but non-fatal shock without tripping the RCD in both cases, and one can envisage lower resistance connections that will trip in both cases. Even when the RCD does trip I imagine that the shock will still be of sufficient duration to make one jump a bit! The RCD is not there to prevent one receiving any sort of shock, it is there to prevent a shock of sufficient intensity to be fatal.

 

I am sorry you are squirming, it is the same fault on the system a resistance on 5000 Ohms a human body. The principle reason for fitting an RCD is to protect the humans that use the system or come into contact with it and if it does not do that it fails.

 

I think you have go yourself too involved in 30mAs and not what the RCD is supposed to do, protect the human users from electric shock. On a boat that level of shock could kill by injuring the human being, throwing them overboard, and probably several other way I cannot think of at the moment.