Touch voltage to earth

[Jako13!]

Can somebody explain if you had a 230v to earth fault the variables that determine touch voltage of a person to earth and is this voltage less the closer you get to the MET of an installation.

Thanks guys

 

[LastManOnline]

Can somebody explain if you had a 230v to earth fault the variables that determine touch voltage of a person to earth and is this voltage less the closer you get to the MET of an installation.

Thanks guys

Well if the line to earth fault takes place in a TNC-S system (the one I am most familiar with) then you are correct in assuming that the closer you get to the MET then the smaller the touch voltage will be.
The basic variables that will determine the touch voltage to earth will be the point of resistance along the new circuit that has been formed by the short, that a person physically comes into contact with.

 

[scotsparky]

max touch voltage at any point of Installation is 50v

Regs book is in van and its ----ing down at moment.

the touch voltage is linked to max currant a human body can take before its considered fatal.
it works out about 1667 Ohms of resistance for a human.
1667Ohms is also max Zs for a 30ma RCBO.

 

[LastManOnline]

max touch voltage at any point of Installation is 50v

Regs book is in van and its ----ing down at moment.

the touch voltage is linked to max currant a human body can take before its considered fatal.
it works out about 1667 Ohms of resistance for a human.
1667Ohms is also max Zs for a 30ma RCBO.

Yes. You are correct regarding the max touch voltage not been over 50v.But I u derstood the OP question was in relation to the "variables" that would determine this voltage. Hope the "-----" has stopped

 

[scotsparky]

Jako its Ohms law.

Yes. You are correct regarding the max touch voltage not been over 50v.But I u derstood the OP question was in relation to the "variables" that would determine this voltage. Hope the "-----" has stopped

max is always 50V no change between end of final circuit and metal main Distribution board over 50V could be fatel.

 

[pc1966]

During a fault you will get the supply volts times R2/(R1+R2) relative to true Earth where R1 includes the supply phase impedance, and R2 includes the supply ground.

But it is complicated by the other aspects such as any water/gas bonding to metallic pipes that might raise the local earth where you are standing up a bit relative to true Earth.

So you can see voltages from around 1/2 supply (on long supply cable with R1 around R2) to close to the supply (if R2 is very high on TT system). But the danger of serious impact also depends on the exposure time, hence the values of 0.4s (typical LV TN-S) and 0.2s (TT) for max disconnection to try and keep the likely current-time exposure down.

If you are unfortnuate enough to be in good contact, say a swimming pool , then the current can be much higher even though a similar disconnection time is likely. Hence the requirement for SELV in that sort of high risk area as an RCD limits the exposure time, not the actual current, you might get.

 

[scotsparky]

During a fault you will get the supply volts times R2/(R1+R2) relative to true Earth where R1 includes the supply phase impedance, and R2 includes the supply ground.

But it is complicated by the other aspects such as any water/gas bonding to metallic pipes that might raise the local earth where you are standing up a bit relative to true Earth.

So you can see voltages from around 1/2 supply (on long supply cable with R1 around R2) to close to the supply (if R2 is very high on TT system). But the danger of serious impact also depends on the exposure time, hence the values of 0.4s (typical LV TN-S) and 0.2s (TT) for max disconnection to try and keep the likely current-time exposure down.

If you are unfortnuate enough to be in good contact, say a swimming pool , then the current can be much higher even though a similar disconnection time is likely. Hence the requirement for SELV in that sort of high risk area as an RCD limits the exposure time, not the actual current, you might get.

Isn’t R2 the CPC of the final circuit measurement. The main earth readings. On TNC-S and TT are linked in relation to disconnection times PFC etc but not measured as an R2

 

[loz2754]

See this attachment for the long answer...

 

[pc1966]

Isn’t R2 the CPC of the final circuit measurement.

Yes, that is the normal convention, which is why I put in the comments about it being the earth return to the source (i.e. local CPC and supply ground combined). Same for R1 being the impedance to the source (local circuit R1 plus supply phase R1).

Basically R2/(R1 + R2) is your classic voltage divider / potentiometer.

The Ze value has both supply phase and supply ground combined but you don't know in what ratio (i.e. is supply R1 similar to supply R2 so both around Ze/2 or not). Though in the TT case the supply is so much less than most rod's Ra values it can largely be ignored. The DB Zs would have any supplementary bonding of service pipes included so normally is less by a small amount (supply R2 having multiple paths back to the source / true Earth).

How much less depends on the extraneous part's path to Earth. In my of flat's case it is noticable as they are shared with other flat's bonding to the supply earth, hence my DB's Ze is 0.19 ohm and Zs 0.16 ohm (84% of Ze so 19% higher PFC).
[automerge]1599988359[/automerge]

See this attachment for the long answer...

That has all of the details and the impact of the extraneous part bonding covered as well. Well done for finding that document!

 

[LastManOnline]

See this attachment for the long answer...

HELP!! Attachment wont download for me. Any advice very welcome.

 

[pc1966]

HELP!! Attachment wont download for me. Any advice very welcome.

Still works for me (Firefox on Linux PC), but I expect any desktop should work. Are you using a phone to view things?

 

[LastManOnline]

Still works for me (Firefox on Linux PC), but I expect any desktop should work. Are you using a phone to view things?

Tablet. But I will give the desktop a go. Thanks ?

 

[littlespark]

It’ll have ended up in some “downloads” folder somewhere. You got to go look for it.

 

[pc1966]

Probably the folder with "beware of the leopard" on it.

 

[LastManOnline]

It’ll have ended up in some “downloads” folder somewhere. You got to go look for it.

Yes. It kept saying download failed but I will take pc1966 advice. Thank you