A good guide For 'exported PME' and outbuildings!

[Engineer54]

A lot of these myths stem from the numerous ''Scaremongers'' of the PME system!! In that ''IF'' (and it's a very big IF) a break in the neutral occurs all connected metal parts will have a potentially dangerous voltage. In reality, the chances of that happening is remote to say the least. I would take a PME system earth out, to any out-building, wouldn't give it a second thought either

Again In reality, what would you rather have as your earth connection, a system that gives you 0.35ohms or a rod at maybe 200ohms, ...i know what i would rather have in my home.

There is no reason why you can't employ a rod on a PME system, in fact many countries insist on it. Many local authorities used to rod TNS supplies to there council houses in the 50s.

 

[Blue Whale]

Many thanks for posting this info.

 

[Engineer54]

Then there's 'if there are extraneous-conductive parts in the outbuilding, you can't export PME.

Then there's 'you must inform the DNO'
Then there's 'you must inform the DNO'

Seems you still haven't quite grasped the point have you. Yes you CAN take PME to an outbuilding with extraneous-conductive parts!! Your not exporting your extending the equipotential zone!! Please read ALL of this thread, or you will be taking the thread back to the very start again with your post above!!

There is NO must inform the DNO, ...about the majority of installations undertaken by electricians, and certainly not in the case of shed in the back garden. Get your facts sorted out please!!!

 

[ElectricalAsh]

thanks

 

[IQ Electrical]

I am an electronics engineering graduate and I know a little about electrical topics.

Oh good, for a moment I thought you were a spammer...

 

[WayneL]

Let's get the terminology right, shall we chaps.

You cannot 'export PME' to outbuildings.

Combining the Neutral and Earth on the consumer side of the installation is not allowed in this country.

You can, however, run an Earth, or Bonding Conductor, from your installation MET to an outbuilding - regardless of the supplied earthing system.
You must though, as Designer and Installer, take into account 'Extraneous Conductive Parts' in the outbuilding.
If it's a PME supplied Earthing System, then you would need to bond these back to the MET, as you would any other Extraneous Conductive Part.
This could mean running a 10mm Bonding to the shed to accompany a 2.5 mm socket/lighting feed.


The other thing that needs to be taken into consideration is
uninsulated flooring in the shed/outbuilding.
This could mean that the floor is at 'true earth' potential - bit like being outside.
This could be dangerous if there was a loss of supply neutral, (or a high resistance joint on the supply neutral), as all bonded metalwork will rise in potential compared to true earth.

So, it's not a case of myths, it's a case of people only grasping bits of information and repeating them.

There isn't a 'yes you can' or a 'no you can't' - it's up to you, the designer/electrician, to make your decision based on the characteristics of the installation and the outbuilding.

Happy Days

 

[Engineer54]

Your not exporting anything, but rather Extending an Extraneous Conductive Zone, in cases where there are extraneous conductive parts at outbuildings, whether it be TN-S or TNC-S (PME)....

As for uninsulated floors, well there's not much difference in this instance, to housing built prior to i think the 60's where there was no slab damp proofing layer required or laid under solid concrete floor slabs and with tiled finishes...

As for the dangers, the same dangers exist at any PME installation, but the chances of that ever happening is to say the very least is slim indeed.... If there was to be a catastrophic neutral breakdown on a PME supply line, the chances are, it will be tripped out of service by the DNOs protective devices at the supply source. In truth, there are as many myths to PME neutral failures, as to being able to extend a PME system!!!

 

[WayneL]

Your not exporting anything, but rather Extending an Extraneous Conductive Zone, in cases where there are extraneous conductive parts at outbuildings, whether it be TN-S or TNC-S (PME)....

As for uninsulated floors, well there's not much difference in this instance, to housing built prior to i think the 60's where there was no slab damp proofing layer required or laid under solid concrete floor slabs and with tiled finishes...

As for the dangers, the same dangers exist at any PME installation, but the chances of that ever happening is to say the very least is slim indeed.... If there was to be a catastrophic neutral breakdown on a PME supply line, the chances are, it will be tripped out of service by the DNOs protective devices at the supply source. In truth, there are as many myths to PME neutral failures, as to being able to extend a PME system!!!

Being an 'Engineer', perhaps you could explain how a broken neutral, or (more likely), a high resistance neutral joint in the street outside your property would trip a DNO protective device.

Let's take a scenario:

If you had a rising copper water main in your outbuilding and this was bonded back to the MET in 10mm to satisfy PME supply conditions.
If, for whatever reason, you then lost the supply neutral (which is your earth), or the impedance of this neutral rose undetected due to a dodgy joint etc, then some of the 'normal operating current' from your property (and possibly some of your neighbours' as well), will find an alternative path back to the transformer.

This path could be the rising main in your outbuilding, which would mean that the 'rising main' and any other metalwork will rise in potential, depending on the amount of current taking this path. (don't forget, the RCD won't trip).

If this happens and you touch any of this metalwork in the outbuilding, you should be ok, because it's all bonded together, so all at the same potential, BUT, if you are in contact with 'true earth' (e.g. through the floor) then you could get a shock.

This could also happen under normal operating conditions, although the current taking this alternative path would be very low, unless you had a fantasticly low Ra on the rising water main.

Like I said in my post, it's up to the designer/installer to weigh up the pros and cons of running earths to seperate buildings - it all depends on the circumstances.

 

[Engineer54]

Being an 'Engineer', perhaps you could explain how a broken neutral, or (more likely), a high resistance neutral joint in the street outside your property would trip a DNO protective device.

Let's take a scenario:

If you had a rising copper water main in your outbuilding and this was bonded back to the MET in 10mm to satisfy PME supply conditions.
If, for whatever reason, you then lost the supply neutral (which is your earth), or the impedance of this neutral rose undetected due to a dodgy joint etc, then some of the 'normal operating current' from your property (and possibly some of your neighbours' as well), will find an alternative path back to the transformer.

This path could be the rising main in your outbuilding, which would mean that the 'rising main' and any other metalwork will rise in potential, depending on the amount of current taking this path. (don't forget, the RCD won't trip).

If this happens and you touch any of this metalwork in the outbuilding, you should be ok, because it's all bonded together, so all at the same potential, BUT, if you are in contact with 'true earth' (e.g. through the floor) then you could get a shock.

This could also happen under normal operating conditions, although the current taking this alternative path would be very low, unless you had a fantastically low Ra on the rising water main.

Like I said in my post, it's up to the designer/installer to weigh up the pros and cons of running earths to separate buildings - it all depends on the circumstances.

DNOs tend to take great care in there PME supplies, employing pretty sophisticated monitoring/protection relay devices. I will in due course enquire what such monitoring devices are normally provided, when i next speak to my DNO friend (Regional Manager) and relay that information back to this thread.... I would imagine it would be in the form of a VIP module (multi function monitoring/protection relay) set to the individual parameters of the system it's connected too..

Don't knock a good rod system, lol!!! ...Depending on it's installation and the type of ground soil, it's not particularly uncommon to be re-paid with a sub 1ohm or thereabouts readings. But very doubtful with the use of these now common short thin rods... And would be totally pointless from an economical view point for a domestic outbuilding/shed!! ...lol!!!!

Yes i do agree with you, it most certainly is upto the designer/installer to decide what is the best solution/method of running PME, or any other type of electrical supply to out buildings.

 

[Deleted member 9648]

It always baffles me why the outbuilding PME handwringers only seem to apply their logic to sheds and garages....it never seems to occur to them that the same perceived hazards presumably will apply to external lights and sockets connected to the PME earth on the main building.

 

[WayneL]

It always baffles me why the outbuilding PME handwringers only seem to apply their logic to sheds and garages....it never seems to occur to them that the same perceived hazards presumably will apply to external lights and sockets connected to the PME earth on the main building.

This is true - the old 'outside tap' is another that springs to mind - when it's run straight through the wall yet bonded inside the house to a PME supply

 

[malcolmsanford]

Strange enough wayne one of the reasons a lot of the older sparks still baulk at the export of the PME was that oulined of the outside water pipe.

In the old Electricity Supply Regulations 1988 which was succeeded by the ESQCR-2008, the old ESC-1988 had a PME section and it recommended that an outside water pipe that was supplied from a pipe through the wall/ground from inside the house should be sleeved.

I suppose old habits die hard with some.

 

[amberleaf]

Sorryabout this Dan / Jason . don’t know where to drop it .

Wordsof Wisdom . Why ?? do I need a Fire Alarm

FireAlarm Systems is used to ProtectLife & Property . Therefore ! Any Company or Person who Owns something of Value has a Valid Interestin installing a Fire Alarm . “ LIFE ! ◄◄

•AccentLighting . Lighting used to Emphasise an Object or Specific Area(s) .

•“Meaning “ Ampere Squared Seconds ( I2 t ) The measurement of HeatEnergy developed within a circuit duringthe fuse’s clearing . It can be Expressed as “ Melting ( I2 t ) or “ Racing ( I2 t ) or the sum of them as “Clearing ( I2 t ) “ ( I )stands foreffective let-through current ( r.m.s. ) Which is squared . & the ( t )stands for time of Operating in seconds.

•PowerLoss ( Cable ) Loss caused by internal cable Impedance . mainly ( I2 R ) Theloss cause heating .

•Continuity: The state of being Whole . – Unbroken .

•Drawing: Block diagram . Asimplified drawing of a system showing major items as blocks . normally used toshow how the system works & the relationship between major items . • Drawing : line Schematic ( diagram) Shows how a circuit works . • Drawing: Plot or Layout . Shows the “ Floor Plan “ . • Drawing: Wiring diagram . Shows how the devices are Interconnected .

 

[amberleaf]

•Impedance(A) Theopposition to current flow in an A.C. circuit ; Impedance includes Résistance (R ) Capacitive Reactance ( X C L ) itis measured in Ohms .

•Inverter : Anitem that changes D.C to A.C.

•Kirchhoff’s Law : (1) The algebraic sum of the currents atany point in a circuit is Zero . (2) The algebraic sum of the product of the current & the impedancein each conductor in a circuit is Equal to the electromotive force in thecircuit .

•Load : (1) Adevice that receives power . (2) The power delivered to such a device . - Load Factor . The ratioof the average to the Peak load over a period . Start with the Basic . - Magnet : A body that produces a magnetic field externalto itself . magnets attract iron particles . -Magnet Field : (1)A magnetic field is said to exist at the point at which a force over &above any electrostatic force is exerted on a moving charge at that point . (2)The force field established by A.C.through a conductor – especially a coiled conductor . -Magnet : Thoseportions of the magnet towards which the external magnetic induction appears toconverge ( South ) or diverge ( North ) - Motor : An apparatus to convert from Electrical to Mechanicalenergy . -Motor Capacitor: A single-phaseinduction motor wit an auxiliary starting windings connected in Series with acondenser for better starting characteristics . - Power Factor : Correctioncoefficient for A.C. power necessary because of changing current & voltagevalues . - Efficiency : The ratio of the Output to theInput . - Electromagnet : A deviceconsisting of a ferromagnetic core & a coil that produces appreciablemagnetic effects only . when an Electric current exists in the coil . - Electromotive force ( emf ) Voltage

 

[amberleaf]

Electricalforce that causes current ( freeelectrons ) to flow or move in an electrical circuit . This unit ofmeasurement is the Volt . - Self-Inductance : Magneticfield induced in the conductor carrying the current . * Electron : The subatomic particle that carriesthe Unit Negative charge of electricity . * Proton :Thehydrogen atom nucleus . it is Electrically positive . - Manufactures’ Conditions: Criteria set forth by materials vendorwithin which they will honour Warranties . - Polyphase Circuits : Circuits running onA.C. & having two or more interrelated voltages . Usually of Equal amplitudes. Phase differences . & periods . etc . if a Neutral conductor exists . Thevoltages referenced to the Neutral conductor are Equal in amplitude & Phase. The common version is that of Three-phase , Equal in amplitude . with Phases( 120º apart ) - Powerapparent :The product of r.m.s.volts times r.m.s.amperes . - Rated : Indicating the Limitsof Operating characteristics for application under specified conditions . - Reactance : (1)The imaginary part of Impedance . (2)The opposition to A.C. to capacitance ( Xc) & Inductance ( XL) - Reactor : A device tointroduce Capacitive or Inductive reactance into a circuit . - Résistance : The opposition in a conductor tocurrent . The real part of Impedance . - Short Circuit : Anoften-Unintended Low résistance path through which current flows around .rather than through . a component or circuit .

 

[amberleaf]

You willget this on Drawings . Apprentices . - Service Cable : The service conductors made up in the form ofa cable . - Service Conductors : Thesupply conductors that extend from the Street Main or Transformers to theService equipment of the premises being supplied .

-Surge : (1)A sudden increase in voltage & current (2) Transient condition . - Starter : An electric controller for accelerating amotor from Rest to NormalSpeed & for stopping the motor . - Starting Winding :Winding in an electric motor used only during the brief period when the motoris starting .

DoNot mix up the twoWords . - Stator/ Starter . ◄◄ - Stator :The portion of a rotating machine that includes & supports the stationary ActiveParts . * Volt :The practical unit of voltage of electromotive force . One volt sends a currentof One ampere through a résistance of One ohm . * Voltage: Voltage is the force . pressure . or electromotive force ( emf ) that causes electriccurrent to flow in an electric circuit . its unit of measurement is the Volt .which represents the amount of electrical pressure that causes current to flowat the rate of One Ampere through a résistance of One ohm . Voltage in anelectric circuit is often considered similar to Water pressure in a pipe orwater system . * Voltage Drop( Vd ) The voltage drop in an electrical circuit is difference between thevoltage at the power source & the voltage at the point at which electricityis to be used The voltage drop ( Loss ) is created by the résistance of theconnecting conductors . * Watt : Theunit of measurement of electrical power or rate of Work ( 756 Watt ) isequivalent to ( 1 hp ) The Watt represents the rate at which power is expendedwhen a pressure of ( 1V ) causes current to flow at a rate of ( 1A ) in a D.C.circuit or in A.C. circuit at Unity ( 100% ) power factor . The number of Watts equals the pressure ( in Volts ) multiplied by thecurrent ( in Amperes )

 

[IQ Electrical]

I think the above was meant for a different thread?

 

[amberleaf]

IQ ELECRRICAL .



Sorry about that . the builder chap who’s working next door to me . has be using a mini digger ?? needless to say he took out some Cables .Internet . I’ve had trouble with the internet . & getting in to the Forum sorry about the scattering on the download . do except my apologies .Amber

 

[IQ Electrical]

IQ ELECRRICAL .



Sorry about that . the builder chap who’s working next door to me . has be using a mini digger ?? needless to say he took out some Cables .Internet . I’ve had trouble with the internet . & getting in to the Forum sorry about the scattering on the download . do except my apologies .Amber

Beware man with digger!

 

[TinyT]

TN-S supply. 6mm swa, 3core, via a 32a MCB from house, to pool plant room outhouse.
There is a gas pipe in the room that is copper but is run in plastic before it comes in. At the moment i have bonded the pipe with a 10mm earth conductor to the CU MET in the outhouse. Is this acceptable??? I know it is a little of topic