# Disconnection time 120 volts vs 150 volts

#### mbrooke

##### Senior Member
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.

#### ptonsparky

##### Senior Member
1 second will work from here.

What are you removing power from? Is there a current level and time at 150 volts?

I know it’s all above my pay scale, but I’m curious what you are attempting to do.

#### mbrooke

##### Senior Member
1 second will work from here.

What are you removing power from? Is there a current level and time at 150 volts?

I know it’s all above my pay scale, but I’m curious what you are attempting to do.
15, 20 and 30 amp branch circuits. At 120 volts to ground the breaker is required to trip at 0.8 seconds or under. At 150 volts to ground 0.4 seconds or under. Sounds like a lot simply going from single phase to three phase.

And don't sell yourself short

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#### mbrooke

##### Senior Member
The time is dependent on the resistance (length) of the 14, 12 and #10 cu.

#### romex jockey

##### Senior Member
I can recall an old farmhouse way out , with a barn wired off of it, to yet another barn in the back 40....

One could arc something all day long, w/out the ocpd opening @ the main house

~RJ~

#### ptonsparky

##### Senior Member
A LONG time ago I did an absolute Minimum with max VD for a short time project. 240v 3Ph, IIRC. Under a bolted fault that developed at the far end, the voltage would drop out the control but never blew a 60 amp fuse.

#### Sahib

##### Senior Member
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.

If you know the short circuit current at 150V, the short circuit rating constant ( K ) of the type of cable, obtainable from cable manufacturers, then you can calculate the area of cable suitable for your requirement. The formula is . Area= (I/K)*√time. Note:The formula applicable upto 5 seconds only.

#### Sahib

##### Senior Member
Ah! The area as expressed in formula in my previous post is in Sq.mm.

#### mbrooke

##### Senior Member
Ah! The area as expressed in formula in my previous post is in Sq.mm.
Sahib, this is what I'm looking for! I have the mm2. Is this enough?

#### mbrooke

##### Senior Member
I can recall an old farmhouse way out , with a barn wired off of it, to yet another barn in the back 40....

One could arc something all day long, w/out the ocpd opening @ the main house

~RJ~

Indeed what I'm trying to avoid. 60 standing volts on the grounding system might be an issue to anyone with low skin conductivity and standing in mud.

#### mbrooke

##### Senior Member
A LONG time ago I did an absolute Minimum with max VD for a short time project. 240v 3Ph, IIRC. Under a bolted fault that developed at the far end, the voltage would drop out the control but never blew a 60 amp fuse.
Yup! You see what I'm trying to avoid (trying to accomplish)

So how do I go about the math? :dunce:

#### Sahib

##### Senior Member
Sahib, this is what I'm looking for! I have the mm2. Is this enough?
Enough for what?

#### mbrooke

##### Senior Member
Enough for what?
For the area of the cable.

Do you do loop impedance in your Country?

#### Sahib

##### Senior Member
For the area of the cable.

Do you do loop impedance in your Country?
I personally never did.

#### mbrooke

##### Senior Member
I personally never did.
Have you ever done the calcs? I'm curious how 1.5, 2.5 and 4mm2 cable will behave.

#### Sahib

##### Senior Member
Have you ever done the calcs? I'm curious how 1.5, 2.5 and 4mm2 cable will behave.
No occasion arose for me to do the calculation because I need only to apply ready made schedule of work for standard electrical works in telephone exchange buildings. But in your case to do the calculation, you need to know also the prospective short circuit current as well as the constant k. Also see the websites mentioned in synchro last post for clarity.
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#### mbrooke

##### Senior Member
No occasion arose for me to do the calculation because I need only to apply ready made schedule of work for standard electrical works in telephone exchange buildings. But in your case to do the calculation, you need to know also the prospective short circuit current as well as the constant k. Also see the websites mentioned in synchro last post for clarity.
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Say I have a 750 kva 5% Z 138/240Y transformer.

5 x 300 mm2- 10 meters to MDP

50mm2 from MDP to panel 50 meters

1.5mm2 to last fitting (j-box) 75 meters

How would you calculate Ze= Z2+R1+R2?

#### mbrooke

##### Senior Member
How does this table look? Is this one way resistance? This factors in reactance by the looks of it?