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02 April 2014 08:24 PM
OMS Posts: 22864 Joined: 23 March 2004	I understand the need for earthing. Good All circuits are earthed and the main earthing conductor completes the earthing of the circuits. The part where I disagree is bringing extraneous-conductive-parts in to the electrical system by placing bonding conductors on them. Think about that again - if they are extraneous, they already have a potential (usually earth potential) - if you touch a metal item with an earth fault and that extraneous conductive part at earth potential you experience a touch voltage - it's what we used to call an indirect contact shock. If I bond that conductor to the MET the fault causing the aplliance case to rise in voltage also pulls up that extraneous part and the voltage between the parts (ie the touch voltage experienced by the person is reduced It is pure theory is it not that this earthing system comprising of earth, main bonding and supplementary bonding makes parts that would not otherwise be part of the electrical system safer if they do inadvertently acquire a voltage through a fault. It most certainly is not theory - it a physical fact - ohms law dictates it It is a well known fact that CURRENT causes death not VOLTAGE. LoL - you'll be telling me next that "it's the volts that jolt but the mills that kill" - it is incredibly difficult to seperate the amps from the volts when you have a resonably fixed impedance like a human body OK - if you want to be a pedant, if I apply a voltage to an impedance, then we will have current - the value of current being dictated by the voltage and the impedance combination. As the human body impedance is broadly fixed, then less or more voltage means less or more current - ohms law again. A voltage can be present on something but current will only flow if a circuit can be made. Indeed, the human body has impedance so the circuit is always made if only by capacitive coupling - why do you think we have limits on systems like Medical IT arrangements that are protecting from micro shock - in those kind of cases we are taking about perhaps a few milliamps in a suppressed patient being fatal 50mA of current is sufficient to kill. It's a lot less than that actually - depends on the person and where we apply it - a few mills will do the job once we are through the skin - don't forget the human body is just a resonably tough bag containing lots of conductive fluids Merely touching a live part if no current can flow will not kill you but you will get a painful shock if the voltage is big enough. You would need to be very highly insulated for no current to flow - and that's not practicable in reality. I suspect Emma Shaw was already experiencing some current flow before she touched earthy metalwork Seriously, you need to research and understand the touch voltage concept - because if you let your current thinking drive your design approach regarding bonding of extraneous conductive parts (or not in your case) then that is truly dangerous. Do not let a little knowledge cause you a problem- take the time to understand the basic science behind the reasons for earthing and bonding - trust me, they are rooted in good science Regards OMS ------------------------- Let the wind blow you, across a big floor.
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02 April 2014 11:38 PM
phantom9 Posts: 1757 Joined: 16 December 2002	Originally posted by: OMS Seriously, you need to research and understand the touch voltage concept - because if you let your current thinking drive your design approach regarding bonding of extraneous conductive parts (or not in your case) then that is truly dangerous. Do not let a little knowledge cause you a problem- take the time to understand the basic science behind the reasons for earthing and bonding - trust me, they are rooted in good science Regards OMS Just to reassure you I do carry out main equipotential bonding! My debate is about trying to justify it. I have GN8 but not GN5. I will add that to my list.
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03 April 2014 02:39 PM
mawry Posts: 334 Joined: 26 April 2004	There's a book by BD Jenkins. Electrical installation calculations which has a nice bit on the Touch Voltage Concept in one of the appendices. You can get the latest version from Amazon for 20 odd quid. Someone also posted on here a few years back a very nice spreadsheet that was very good at indicating the relationships between V, I and R1, R2 etc.
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06 April 2014 06:09 PM
briandoherty Posts: 313 Joined: 08 May 2004	Originally posted by: mawry There's a book by BD Jenkins. Electrical installation calculations which has a nice bit on the Touch Voltage Concept in one of the appendices. You can get the latest version from Amazon for 20 odd quid. Someone also posted on here a few years back a very nice spreadsheet that was very good at indicating the relationships between V, I and R1, R2 etc. I guess there's a chance that the spreadsheet referred to is the one I spent rather more of my employer's time on than I ought to have done when there was money to be earned elsewhere....brian's fault current / touch voltage spreadsheet ------------------------- Regards, Brian
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07 April 2014 07:27 PM
Zs Posts: 3877 Joined: 20 July 2006	http://www.dailymail.co.uk/new...fined-just-1-000.html Daily Mail article for your information. Zs
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08 April 2014 11:13 AM
mawry Posts: 334 Joined: 26 April 2004	That's the one Brian, nice work!
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08 April 2014 07:46 PM
mikejumper Posts: 2810 Joined: 14 December 2006	If testing had been carried out properly would it have picked up this fault? If the screw just clipped the live conductor on it's way into the metal studwork, thereby connecting the two together, how would any of the insulation tests we usually do show this?
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09 April 2014 12:06 AM
napitprofessional Posts: 452 Joined: 08 March 2008	Originally posted by: mikejumper If testing had been carried out properly would it have picked up this fault? If the screw just clipped the live conductor on it's way into the metal studwork, thereby connecting the two together, how would any of the insulation tests we usually do show this? In this case, insulation testing would have found the fault instantly. It was self-evident, therefore, that the testing had not been done - even though the (apparently) "industry standard" value of 200 meg was entered onto the forms, albeit in the wrong column. Clearly, Anchor Building and Electrical (in line with so many others) subscribed to the ongoing notion that lack of measured test results should not stand in the way of producing a completed certificate .... I wonder when the next "one-off" will occur? ------------------------- B. Eng (Hons) MIET
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09 April 2014 10:50 AM
Zoro Posts: 300 Joined: 31 July 2011	Nobody should take comfort at the thought of not testing, in the case of Emma Shaw, that maybe the fault may not have been found by testing to BS7671. By forensic examination and evidence given in court, the screw was touching both live and cpc, which IR tests before energisation would have found. After the incident the IR tests carried out, gave a reading of 0.2M ohm, even when the screw threads had been blown away by the fault current. No dead test were carried out, and test results were made up, and entered onto the certificate by the Non competent, Unqualified Certifier. The QS certified that he had checked the NICEIC certificate, but it had multiple errors. The QS has a personal responsibility to supervise but he did not, he has a personal responsibility to check the certificate he did not. Part P Schemes are based on the flawed NICEIC QS system, they do not require individual Competence I believe this was a contributing factor. The fact that the NICEIC are fighting to retain this system on the basis that it would be to expensive is disgusting, especially when they are owned by the Electrical Safety Council, or whatever they are calling themselves this week. It was good that the Select Committee rejected this propaganda and told the Schemes to comply with having individual Competence. The only expense would be to contractors that charge for an Electrician and don't supply one. .
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09 April 2014 02:01 PM
napitprofessional Posts: 452 Joined: 08 March 2008	How will the public react to being charged more for what they already think they are paying for and getting ..... ------------------------- B. Eng (Hons) MIET
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