Confused about Zs 5 second disconnection time

[pc1966]

Right, but you're leaving out the source impedance regarding touch potential.

No, the source impedance is part of what determines the PFC, and that in turn with "R2" determins what the touch voltage is.

So for a fixed R2 and the supply Z increases, so does the PFC drop, and then PFC*R2 drops as well.

 

[Julie.]

Touch potential can be quite different to what you may think.

Let's say you have a transformer and supply cable to a property, say the neutral conductor is 0.25 ohm to the star point and the line plus winding is 0.75 ohm, then within the property the circuit r1+r2 is 1.3 ohm.

In the event of a solid fault this would be 2.3 ohm and at 230v would be 100A fault current.

Take the case of a TT arrangement, the local earth obtains its connection via the ground/earth itself from the star of the transformer.

So, the neutral point at the property would rise to 100A x 0.25 ohm = 25V (to earth)
The line would drop to 100A x 1.3 ohm = 130V ABOVE the neutral so 155V (to earth)

Now take the case of a TN-C-S, in this case the combined neutral/earth conductor is usually tied to earth locally via connections to gas/water pipes, multiple earth rods within the supplier network etc.

So even though there is still 25V dropped in the neutral supply cable, the actual local earth is very close electrically to the supply cable at the property NOT the end at the transformer's star point.

So the neutral point at the property would remain close to zero volts measured to the local earth
The line would again go to 130V above neutral, but this time that's 0V so we only see 130V to local earth.

In practice the figures are obviously smaller - I just picked nice figures to illustrate it.

The multiple connections between local earth and the combined neutral/earth connection (PEN) result in lower resistances, and the holding of local earth close to that of the PEN local to the properties.

In the US most properties are similar to the TT system here - even if it has a utility supplied earth, it's often an additional conductor ( TN-S) so very much like TT in terms of this volt drop calculation.

It's the main reason we use the term earth and not ground, ground would be the ground at the substation star point/ground rods - in the US this would be a similar point in terms of volt drop at the property when there is a fault; but in the uk and much of Europe the local ground would be tied close to the point where the PEN is separated; so not the same point as the transformer star. Hence the differential between ground (substation) and earth.

 

[Cookie]

No, the source impedance is part of what determines the PFC, and that in turn with "R2" determins what the touch voltage is.

So for a fixed R2 and the supply Z increases, so does the PFC drop, and then PFC*R2 drops as well.

Right, supply impedance determines PFC. Same applies to touch voltage.





The lower the transformer Z the higher voltage at its output for a short circuit inside the building.

 

[Cookie]

Touch potential can be quite different to what you may think.

Let's say you have a transformer and supply cable to a property, say the neutral conductor is 0.25 ohm to the star point and the line plus winding is 0.75 ohm, then within the property the circuit r1+r2 is 1.3 ohm.

In the event of a solid fault this would be 2.3 ohm and at 230v would be 100A fault current.

Take the case of a TT arrangement, the local earth obtains its connection via the ground/earth itself from the star of the transformer.

So, the neutral point at the property would rise to 100A x 0.25 ohm = 25V (to earth)
The line would drop to 100A x 1.3 ohm = 130V ABOVE the neutral so 155V (to earth)

Now take the case of a TN-C-S, in this case the combined neutral/earth conductor is usually tied to earth locally via connections to gas/water pipes, multiple earth rods within the supplier network etc.

So even though there is still 25V dropped in the neutral supply cable, the actual local earth is very close electrically to the supply cable at the property NOT the end at the transformer's star point.

So the neutral point at the property would remain close to zero volts measured to the local earth
The line would again go to 130V above neutral, but this time that's 0V so we only see 130V to local earth.

In practice the figures are obviously smaller - I just picked nice figures to illustrate it.

The multiple connections between local earth and the combined neutral/earth connection (PEN) result in lower resistances, and the holding of local earth close to that of the PEN local to the properties.

In the US most properties are similar to the TT system here - even if it has a utility supplied earth, it's often an additional conductor ( TN-S) so very much like TT in terms of this volt drop calculation.

It's the main reason we use the term earth and not ground, ground would be the ground at the substation star point/ground rods - in the US this would be a similar point in terms of volt drop at the property when there is a fault; but in the uk and much of Europe the local ground would be tied close to the point where the PEN is separated; so not the same point as the transformer star. Hence the differential between ground (substation) and earth.

I'd have to disagree. If the person is outside in the back lot, on wet grass, the person will see those 25 volts drop across the neutral in addition to the internal R2 volt drop at the outside socket. A local earth rod does not change that unless you are standing on the earth rod itself.


In terms of being inside the building the earth rod again makes no difference when everything is bonbded to the MET. The 25 volts are only eliminated via bonding of pipes and rebar to the MET, not earth rod connected to the MET.