Why are main bonding conductors so big?

[davetheglitz]

It strikes me that main bonding conductors need to take a fair bit of current to allow for exteraneous currents coming in on say the gas or water pipe. I wish the ESC reflected this - as there is no way to justify increasing bonding conductor size by the cooker fault shown in the ESC leaflet on earthing and bonding.

Somehow or other the exteraneous current source would be fused - so there must be a design current that the bonding needs to take before the fuse blows and makes the pipe safe.

Does anyone have any idea what current this is and how it was arrived at - and what has changed so we now need 10mm rather than 6mm - I've never seen a burnt bonding conductor that is not down to a plumber!

 

[steptoe]

the big problems with MEBs is PME earthing, or to be more precise, TNCS pretending to be PME,

you could in theory pull in a fault from numerous other properties in your street and they would all have to travel down your earthing conductor, hence why it is so big,

ie, you have the first house in the street, the neutral breaks just after your T off point,

where is all the current from the remainder of the street going to go?

problem solved if only proper PME with localised rods and every joint spiked,

 

[davetheglitz]

As always Steps - a new insight! I always envisaged a fault with someone digging the road and shorting a cable to another service - so assumed there would be some fusing somewhere. What you have come up with is far nastier!

If I understand correctly what you are saying is that depending on what was on in the street at the time of the neutral break there would be a number of currents flowing, none of which would cause a fuse to blow. Say 15 houses in the street at tea time - could be 15 x 20a - 300A. On that basis is 10mm enough?

Sometime an assumption has been made as to what a reasonable risk is and a decision made to go from 6mm to 10mm. Who makes these assumptions - and do we know what assumptions have been made!

 

[ADS]

The mind boggles

 

[steptoe]

As always Steps - a new insight! I always envisaged a fault with someone digging the road and shorting a cable to another service - so assumed there would be some fusing somewhere. What you have come up with is far nastier!If I understand correctly what you are saying is that depending on what was on in the street at the time of the neutral break there would be a number of currents flowing, none of which would cause a fuse to blow. Say 15 houses in the street at tea time - could be 15 x 20a - 300A. On that basis is 10mm enough?

Sometime an assumption has been made as to what a reasonable risk is and a decision made to go from 6mm to 10mm. Who makes these assumptions - and do we know what assumptions have been made!

thats about right Dave,

this is why I dont like TNCS too much,

and how do you think the DNO are going to solve this before all TNCS have to be PME by August (I think it is) this year?

simples, by simply going round all the supplies and putting a little PME sticker on the service head!!!

 

[apprentice87]

Hi All,

It gets better than that!!! Here goes for another of my weird newbie theories!!!!!!! [Tin helmet on!!!]

Now then; Lets say you have a "multiple occupancy" steel framed building, say a portal framed jobbie with ten units in it.

Now then..... The DNO will only provide a TNCS supply to ONE unit, all the others will have to be TT'ed.

Now, why is this then????? Well, think about this....Say they were all on TNCS...

So, the neutral is connected to the MET in each unit by the earthing conductor, and the MET is bonded to the steel frame.

Now, what is going to happen when all is up and running?????

The current will arrive at any particular unit down the relevant phase conductor. BUT, it will NOT necessarily go "back down" the corresponding units neutral. What will happen, [remember now, neutral is connected to the MET which is bonded to the steelwork] is the neutral current will travel through the steelwork of the building until it finds the neutral conductor with the lowest impedance, and it will go down there instead.

You could end up with the power from ALL the units trying to return down the one neutral. Result, one cooked earthing conductor from the MET to the neutral block, and one cooked neutral in the service cable belonging to whichever unit has the lowest impedance neutral [the one with the shortest service cable i guess]

Now then, back to our street of houses.... The whole street will have their neutrals connected to their MET's which are in turn bonded to the metallic water pipe in the street.

As the various neutrals will have various impedances, and at various times will have different volt drops across them [depending on the load they are carrying], the current trying to make it back to the transformer will "pick and choose" which neutral it wants to return down.

Whatever happens, it is EXTREMELY likely, a positive certainty in fact, that someones elses current will, at some time or other, be coming along the water pipe, popping up into your protective bonding conductor, travelling along it to your MET, down your earthing conductor to the neutral block and back to the LV main..

Hence, it is entirely possible that YOUR bonding conductors will at various times be acting as a return for several properties, which is why the DNO make you have an increased conductor size on TNCS.... [cos they know all this!!!!]

Daft system or what.....

john...

 

[ADS]

Ahh, that's what I love about this forum - when things get explained to us properly..........so, it's 10mm bonding so that it can carry the current from the rest

of the street when the supply neutral breaks.

I wish they'd just told us that during training.....it's so simple when you think about it. :Salute

As I said, the mind boggles!

 

[apprentice87]

Hi There,

Not just when the supply neutral breaks, your bonding could be carrying other users diverted neutral currents under fault free conditions too...

Remember, everyone's neutral is connected to everyone elses really, when you think about it, through the bonding conductors, earthing conductors, and any shared metal services, so whichever lucky person just happens to have the lowest impedance neutral, will end up carrying currents for everyone else, under normal fault free conditions too...

john...

 

[ADS]

Are my kids safe in the bath then?.......I've got metal taps.

And what about their electric toothbrush??

 

[sellers]

Hi There,Not just when the supply neutral breaks, your bonding could be carrying other users diverted neutral currents under fault free conditions too...

Remember, everyone's neutral is connected to everyone elses really, when you think about it, through the bonding conductors, earthing conductors, and any shared metal services, so whichever lucky person just happens to have the lowest impedance neutral, will end up carrying currents for everyone else, under normal fault free conditions too...

john...

How would you possibly carry other peoples current? Why would the current divert from the road upto your neutral in your house? That wouldn't be the path of least resistance? Unless I'm missing something!

Interesting thread.

 

[ADS]

Interesting thread.

D'you think?? ; \

 

[steptoe]

How would you possibly carry other peoples current? Why would the current divert from the road upto your neutral in your house? That wouldn't be the path of least resistance? Unless I'm missing something!Interesting thread.

he's taking about nissin huts,

you know, the tin towns they built for 'temporary' accommodation after the war! (whisper)

trust John/app87 to get all twisted and complicated on us, but yes, in theory it could happen,

have I ever told you?

I DONT like TNCS

 

[apprentice87]

Hi Sellers,

Say you live next door to me... your incoming neutral/earth is connected to your MET by your earthing conductor. Your MET is connected by your protective bonding to the water pipe, mine is exactly the same too. BUT, my service cable has a lower impedance than yours, guess which way your neutral currents are going to flow...

This is why the DNO WILL NOT provide "PME" to steel framed buildings with more than one supply, because there is no way of knowing which route home the neutral currents will take.

Where is the difference between a steel framed building with all separate supplies all bonded to the same steel frame, and a row of houses with separate supplies all bonded to a steel pipe..

john

 

[steptoe]

this, I feel is why the DNOs are starting to feel it,

previously they seemed to be a bit lax and its almost as if they were using the incoming services as their localised rods,

[they wouldnt do that would they?]

now, with the majority of services in plastic, they are being forced to go and put little stickers on folks service heads,

those magic stickers will be better than any rod they cant be bothered to put in.......

 

[sellers]

Hi Sellers,Say you live next door to me... your incoming neutral/earth is connected to your MET by your earthing conductor. Your MET is connected by your protective bonding to the water pipe, mine is exactly the same too. BUT, my service cable has a lower impedance than yours, guess which way your neutral currents are going to flow...

This is why the DNO WILL NOT provide "PME" to steel framed buildings with more than one supply, because there is no way of knowing which route home the neutral currents will take.

john

Next door may have a lower impedance on their neutral but i highly doubt the impedance would be less by the time it goes through my bonding and water pipe through their bonding and down their neutral? Therefore it would go down my neutral.

I suppose it could be common in houses with back to back service heads.

 

[Technician]

As I go around the country I am surprised by the

number of overhead supplies that are TN-C-S that

were put in between WW parts 1 and 2.

I can only assume that they were not buried because

of the speed at which the houses were thrown up,

unless there was another reason for overhead rather

than buried, like aggressive soil conditions.

 

[Zeespark]

Tech, cost, ALOT cheaper to clip cables at a ten foot height or so between houses as opposed to burying them.

AndyGuinness

 

[Technician]

Zee, that is how I looked at it; I wondered

if there were any other reasons. Housing

development in some parts of the country

was so rapid that overhead was possibly

THE only solution.

 

[Zeespark]

Not only that Tech, but the DNO's would absolve themselves of any issues if a customer was to install their own earthing system.

AndyGuinness

 

[Technician]

That's a good one; would that include the

one about the metallic object in a bath

of water?