Disconnection time 120 volts Uo vs 150 volts Uo

[Tony S]

120V or 230V to earth is the norm throughout the world. I'm not sure about Outer Mongolia.

 

[ProMbrooke]

likewise, sounds like foreign places to me

I'm talking about Uo. On a 138/240 volt Y system Uo is 138 volts. 150 volts at the high end of the 240 volt nominal (260 volts max) supply tolerance.

 

[ProMbrooke]

120V or 230V to earth is the norm throughout the world. I'm not sure about Outer Mongolia.

Not necessarily. Philippines, Saudi Arabia, and parts of South America derive 230 volts line to line.

 

[Sidewinder]

Firstly with faults. It is the U0 Voltage that we are concerned with.

How are you determining the disconnection time, is this a theoretical exercise, a design or calculation, or is it a measured value which when compare to the breaker impedance is giving these disconnection times?

At the instant of the fault the supply live conductor and the protective conductor act as a voltage divider.

To simplify things, lets allow them to be of equal value for a moment, this means at the instant of fault you are dropping half U0 in the line & half U0 in the protective conductor.

The exact values will vary depending on the exact impedances, but for this model 50:50 will do.

So, with U0=120V this gives 60V at the point of the fault, with U0=150V you have 75V.

From this you can look back at the tables and data in IEC 61140 & IEC 61479 and derive the allowable disconnection times.

These times are related to the physiology of the body, expected heart rate, body impedance etc. For the population based on statistical models.

 

[ProMbrooke]

Firstly with faults. It is the U0 Voltage that we are concerned with.

How are you determining the disconnection time, is this a theoretical exercise, a design or calculation, or is it a measured value which when compare to the breaker impedance is giving these disconnection times?

At the instant of the fault the supply live conductor and the protective conductor act as a voltage divider.

To simplify things, lets allow them to be of equal value for a moment, this means at the instant of fault you are dropping half U0 in the line & half U0 in the protective conductor.

The exact values will vary depending on the exact impedances, but for this model 50:50 will do.

So, with U0=120V this gives 60V at the point of the fault, with U0=150V you have 75V.

From this you can look back at the tables and data in IEC 61140 & IEC 61479 and derive the allowable disconnection times.

These times are related to the physiology of the body, expected heart rate, body impedance etc. For the population based on statistical models.

Well put, my exact thinking. But what specifically am I looking for in IEC-61140 and IEC-61479?

 

[ProMbrooke]

120V or 230V to earth is the norm throughout the world. I'm not sure about Outer Mongolia.

I measured the voltage today, got 122.4 volts line one to earth, 122.5 volts line two to earth, and 244.5 - 244.7 volts line to line. 

https://imgur.com/a/UhawBsS

https://imgur.com/a/evv95Pn

https://imgur.com/a/JyyTdsJ

https://imgur.com/a/9sx1lWQ

What would you say?

 

[Sidewinder]

OK, IEC standards are not concerned with actual measured voltages, they are based on the nominal voltage as stated in the supply characteristics from the supply "authority".

If you are going to take on the liability for designing outside the specified disconnection times, then your RA had better be pretty robust as well as your PII able to take a good kicking to keep you out of trouble if things go wrong!

To start with both fault voltages are classed as lethal in IEC documents.

However, you will need to look at the body model and apply the calculations for the fault current, then apply this to the statistical model for human body resistances in the most onerous conditions you expect then calculate the current likely to flow through the body of the person, then assess the physiological affect on said person from that current flow, then refer back to the model to look at what percentile of the population are likely to be harmed or killed.

Then you an try and justify your deviation for elongating the disconnection time.

Good luck.

Oh & btw, this isn't a fully formed answer, it's just a suggestion as to how you may estimate this.

If you want to follow this through and develop a whole formal risk assessment to justify this action, this post isn't adequate for that.

It can be done & I can advise, but not on an internet forum.

Do I think it's a good idea to stretch the disconnection time, no, would I recommend it or say that it is acceptable, never.

 

[ProMbrooke]

I know, but I want to prove that I am indeed dealing with 120 and 138 volts to ground instead of 230 volts to ground.

 

[Sidewinder]

That should be obvious from your supply characteristics.

I am guessing that you are Cookie then?

 

[ProMbrooke]

Yes- also posted on another forum.

 

[ProMbrooke]

Do I think it's a good idea to stretch the disconnection time, no, would I recommend it or say that it is acceptable, never.

Yes, but remember that it means more copper. Having to run 4mm2 where 2.5mm2 will do is a big incentive to use a longer disconnect time.

 

[binky]

so are you happy to possibly kill people to save money?  

 

[Sidewinder]

Yes, but remember that it means more copper. Having to run 4mm2 where 2.5mm2 will do is a big incentive to use a longer disconnect time.

Nope I have to disagree with that statement.

If it was perhaps 2.5 mm sq. instead of 195mm sq. perhaps, but the specific difference in cost between 2.5 & 4 is negligible.

The difference is that it's not the actual voltage that is considered in this under IEC requirements nut the nominal one.

Thus the voltage divisor is already allowed for in IEC considerations.

 

[Andy™]

Yes, but remember that it means more copper. Having to run 4mm2 where 2.5mm2 will do is a big incentive to use a longer disconnect time.

basically use a longer disconnection time to save a few £ in cable...

 

[ProMbrooke]

so are you happy to possibly kill people to save money?  

Thats why I'm referencing the IEC's body graph and asking here. 0.8 to 0.4 is a big jump.

Nope I have to disagree with that statement.

If it was perhaps 2.5 mm sq. instead of 195mm sq. perhaps, but the specific difference in cost between 2.5 & 4 is negligible.

The difference is that it's not the actual voltage that is considered in this under IEC requirements nut the nominal one.

Thus the voltage divisor is already allowed for in IEC considerations. 

I know the IEC us using a voltage divider, and that the nominal voltage can be +10%.

However, 0.8 seconds for a 120 volt Uo and 0.4 seconds for a 138 volt Uo is a big jump. Why not 0.75 or 0.6 at most?

Cost adds up in big projects not to mention its harder to work with and terminate 4mm2 on a socket terminal.

 

[ProMbrooke]

basically use a longer disconnection time to save a few £ in cable...

Yes

But also remember I'm dealing with 138/240Y, and not 230/400Y. The voltage to remote earth is way lower. 

 

[Sidewinder]

Thats why I'm referencing the IEC's body graph and asking here. 0.8 to 0.4 is a big jump.

I know the IEC us using a voltage divider, and that the nominal voltage can be +10%.

However, 0.8 seconds for a 120 volt Uo and 0.4 seconds for a 138 volt Uo is a big jump. Why not 0.75 or 0.6 at most?

Cost adds up in big projects not to mention its harder to work with and terminate 4mm2 on a socket terminal.

Normally the volt drop gets you way before the loop impedance anyway, especially since the limits were tightened up.

 

[Sidewinder]

Probably because of this:

 

[ProMbrooke]

Normally the volt drop gets you way before the loop impedance anyway, especially since the limits were tightened up.

How much does code limit voltage drop?

Probably because of this:

View attachment 10345

I'm confused, where does this say 75 volts for 0.8 seconds at a tpyical body impedance results in burns or ventricular fibrillation? 

 

[Sidewinder]

I'm confused, where does this say 75 volts for 0.8 seconds at a tpyical body impedance results in burns or ventricular fibrillation? 

It gives typical body resistance for voltage levels, the lower the resistance the greater the current, as you can see it’s not linear.