Jump to content

DONT STAY LURKING AS A GUEST,

JOIN TEF AND BENEFIT FROM MEMBERSHIP:

CLICK HERE TO JOIN.
  • 0
ProMbrooke

Disconnection time 120 volts Uo vs 150 volts Uo

Question

ProMbrooke

Is there any real danger in using a 0.8 seconds as disconnection time for 150 volts line to ground? Or would a sweet spot at 0.7 seconds work? Call me cheap but I don't want to use 0.4 seconds unless I really have to.
 

 

 

Share this post


Link to post
Share on other sites

Recommended Posts

  • 0
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.

 

 

Share this post


Link to post
Share on other sites
  • 0
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.

Share this post


Link to post
Share on other sites
  • 0
Sidewinder

That should be obvious from your supply characteristics.

I am guessing that you are Cookie then?

 

Share this post


Link to post
Share on other sites
  • 0
ProMbrooke

Yes- also posted on another forum.

Share this post


Link to post
Share on other sites
  • 0
ProMbrooke
On 30/11/2019 at 14:20, Sidewinder said:

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.

Edited by ProMbrooke

Share this post


Link to post
Share on other sites
  • 0
binky

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

Share this post


Link to post
Share on other sites
  • 0
Sidewinder
15 hours ago, ProMbrooke said:

 

 

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.

Share this post


Link to post
Share on other sites
  • 0
Andy™
18 hours ago, ProMbrooke said:

 

 

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...

Share this post


Link to post
Share on other sites
  • 0
ProMbrooke
22 hours ago, binky said:

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.

6 hours ago, Sidewinder said:

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.

Share this post


Link to post
Share on other sites
  • 0
ProMbrooke
4 hours ago, Andy™ said:

 

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. 

 

Share this post


Link to post
Share on other sites
  • 0
Sidewinder
32 minutes ago, ProMbrooke said:

 

 

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.

Share this post


Link to post
Share on other sites
  • 0
Sidewinder

Probably because of this:

image.png

Share this post


Link to post
Share on other sites
  • 0
ProMbrooke
21 hours ago, Sidewinder said:

 

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?

21 hours ago, Sidewinder said:

Probably because of this:

image.png

 

 

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

Share this post


Link to post
Share on other sites
  • 0
Sidewinder
38 minutes ago, ProMbrooke said:

 

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.

 

Share this post


Link to post
Share on other sites
  • 0
Sidewinder

image.png

Share this post


Link to post
Share on other sites
  • 0
ProMbrooke
1 hour ago, Sidewinder said:

 

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

 

 

 

I know, but nothing says that subjecting a person at 500 ohms body resistance to 75 volts for 0.8 seconds causes dangerous physiological responses.

1 hour ago, Sidewinder said:

image.png

 

 

Thanks :)

Share this post


Link to post
Share on other sites
  • 0
Sidewinder
2 minutes ago, ProMbrooke said:

 

 

I know, but nothing says that subjecting a person at 500 ohms body resistance to 75 volts for 0.8 seconds causes dangerous physiological responses.

 

 

Thanks :)

Work out the current flowing 10mA across the heart is considered fatal, this hand to hand.

Share this post


Link to post
Share on other sites
  • 0
ProMbrooke
Just now, Sidewinder said:

Work out the current flowing 10mA across the heart is considered fatal, this hand to hand.

 

 

But you have time as well.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

Terms of UseWe have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.