# Maximum Disconnection Times

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#### Cookie

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What is the reasoning behind having a 0.8 disconnection time for up to 120 volts Uo, but it then jumps to 0.4 seconds as soon as you go over it? Wouldn't it be better 0.8 seconds for 150 volts Uo and under, 0.4 seconds 150 volts and over Uo?

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#### davesparks

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What is the reasoning behind having a 0.8 disconnection time for up to 120 volts Uo, but it then jumps to 0.4 seconds as soon as you go over it? Wouldn't it be better 0.8 seconds for 150 volts Uo and under, 0.4 seconds 150 volts and over Uo?
We don't use 150V supplies, but we do have 110 and 230 so in reality whether you set the limit at 120 or 150 the real world effect will be the same.

What is your logic behind saying its better at 150V rather than 120V?

#### Cookie

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When using a single phase supply you get 120 volts line to earth, but on a 3 phase supply you get 138 volts to earth which increases the disconnection time requirements (ie faster trip mandated).

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#### Megawatt

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Arms
When using a single phase supply you get 120 volts line to earth, but on a 3 phase supply you get 138 volts to earth which increases the disconnection time requirements (ie faster trip mandated).
Cookie I got to be the one but are you asking a question or telling us. Nobody really knows how to respond to the 3 post you have made already.

#### Cookie

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Asking for the codes reasoning mainly. And if changing it will violate the IEC's body graph.

#### Megawatt

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Arms
Asking for the codes reasoning mainly. And if changing it will violate the IEC's body graph.
Ok sorry I can’t help you have a good day

#### Cookie

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Well, the NEC does frequently talk about 150 volts to ground over or under. Would be nice to harmonize the IEC with the NEC.

#### davesparks

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When using a single phase supply you get 120 volts line to earth, but on a 3 phase supply you get 138 volts to earth which increases the disconnection time requirements (ie faster trip mandated).
No we don't, standard single phase and three phase supplies in the UK are 230V to earth.

The 120V would only usually be relevant to a reduced low voltage supply on construction sites.
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Well, the NEC does frequently talk about 150 volts to ground over or under. Would be nice to harmonize the IEC with the NEC.
That will be pretty difficult with the big differences between electrical installations in different countries.

#### Cookie

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No we don't, standard single phase and three phase supplies in the UK are 230V to earth.

The 120V would only usually be relevant to a reduced low voltage supply on construction sites.
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Yup

That will be pretty difficult with the big differences between electrical installations in different countries.

Usually, but in places like the Philippines, Saudi Arabia and parts of Latin America one is often left working with both codes. An NEC system with IEC standards applied to them.

See page 8:

120/240 and 138/240Y is found in older installations that have been changed from 120 to 230 volt distribution equipment. Makes for some interesting stuff.

#### static zap

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What is the reasoning behind having a 0.8 disconnection time for up to 120 volts Uo, but it then jumps to 0.4 seconds as soon as you go over it? Wouldn't it be better 0.8 seconds for 150 volts Uo and under, 0.4 seconds 150 volts and over Uo?
Think of it as Time and "energy Received" , cooking a resistor .
..Where the resistance is a human wishing to survive an electric shock...
Simplify things thinking the first 50V is wasted getting through your skin !
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Think of it as Time and "energy Received" , cooking a resistor .
The reality is we are saving equipment from fire risk , limiting total amount of energy received - is how we stay safe !

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#### Cookie

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Yes, but 0.8 and 0.4 seconds is a big leap. 0.65 might be nice sweet spot.

#### Cookie

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Why the disagrement on post #3?

Arms
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#### Cookie

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How or why are they incorrect...?

#### DPG

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Arms
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How or why are they incorrect...?
Single phase to earth. Although are you talking US voltages? Your posts seem to have been asking about UK systems.

#### Cookie

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Center tapped 240 is 120. Line to line 240 produces 138.

Republic of the Philippines uses this system.

Yes, BS7671 and IEC codes on how they would go about dealing with those two voltages.

I'm well aware line-line 240 is rare in Europe.

#### DPG

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Center tapped 240 is 120. Line to line 240 produces 138.

Republic of the Philippines uses this system.

Yes, BS7671 and IEC codes on how they would go about dealing with those two voltages.

I'm well aware line-line 240 is rare in Europe.
Err OK then.

#### Cookie

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Its ok. Just this forum is to quick to assume things.

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#### Megawatt

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Arms
Center tapped 240 is 120. Line to line 240 produces 138.

Republic of the Philippines uses this system.

Yes, BS7671 and IEC codes on how they would go about dealing with those two voltages.

I'm well aware line-line 240 is rare in Europe.
In a star connection phase to phase is 240 vac phase to earth 120vac the voltage you read can be as high as 130vac the same goes with 3 phase. It’s a plus or minus 5% on the voltage in the US

#### Cookie

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138-139 volts, at 252 volts (+5%) it is 145 volts.

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#### Megawatt

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Arms
138-139 volts, at 252 volts (+5%) it is 145 volts.
That highest voltage I’ve seen is 130 to ground and where are you getting 138vac
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That highest voltage I’ve seen is 130 to ground and where are you getting 138vac
Your drawings are irrelevant that’s not telling us nothing

#### Cookie

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130 volts would be the highest in a 120/208 system, not a 240 volt wye system.

The drawings are correct for the system being depicted.

#### static zap

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Also got a post deleted in another thread for saying the stuff on Amazon made by China is often not safe to install.
Tricky tight rope - Negative opinions ..
( I'm older and just love moaning )
You get what you pay for, (and some local wholesalers risk a big hit , when they have to honour their returns policy)
Need to educate customers to spend more!

#### Cookie

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Education is key. Which is why I spoke up. But shady manufacturers/sellers know people buy solely based on cost.

#### Megawatt

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Arms
130 volts would be the highest in a 120/208 system, not a 240 volt wye system.

The drawings are correct for the system being depicted.
On paper is one thing, in the real world it’s a totally different world. Why the emphasis on exactly 138vac

#### Cookie

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Due to Uo (138 volts) changing the disconnect time requirements.

#### Megawatt

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Arms
Where's my post gone? I was sticking up for the forum!!!
I’m from the US and we don’t have those kind arrangements

#### DPG

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Arms
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I’m from the US and we don’t have those kind arrangements
I think there's some confusion between systems.

#### Megawatt

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Arms
I think there's some confusion between systems.
DPG confusing is an understatement I don’t know if the OP is asking a question or trying to teach us. LOL

#### Cookie

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Asking about the code's reasoning. And Megawatt- I fully get you!

#### Megawatt

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Arms
Asking about the code's reasoning. And Megawatt- I fully get you!
Cookie we can sit here all day and pull reference form the BS 7671 and posting table 710 which is in the medical locations. What’s that about

#### Cookie

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710? I had 41.1 in mind. I'm curious how those voltage cutoff values were derived.

#### Megawatt

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Arms
710? I had 41.1 in mind. I'm curious how those voltage cutoff values were derived.
Yes I missed table 41.1 maximum disconnection times on page 55 as far as how are the voltages are derived starts with AC and DC theory
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Yes I missed table 41.1 maximum disconnection times on page 55 as far as how are the voltages are derived starts with AC and DC theory
I’m trying to learn the UK ways of doing electrical work

#### Cookie

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Right, but for AC we have 120 volts and under at 0.8, 0.4 for 120 volts and over. Is any real harm granted in having 0.8 cover up to 150 volts?

#### Cookie

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UK wiring requires that general socket and lighting circuits trip a breaker in a required amount of time based on the line to earth voltage.

#### Megawatt

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Arms
UK wiring requires that general socket and lighting circuits trip a breaker in a required amount of time based on the line to earth voltage.
We can test breakers or RCDs but we can’t fix them if there are 1/10th of a second to slow, as far as I go if they trip I’m happy. I read your profile and I applaud you for wanting to learn but don’t just post things you are reading and asking why. I’m here to help anyone that needs it if I can. I’m going to ask you a question are you from the UK or US
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Asking about the code's reasoning. And Megawatt- I fully get you!
If you are looking for reasons why the code wants this or that is something every body wants. You have hundreds of people who make up rules and regulations in the BIS 7671 and they all have different opinions

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#### Cookie

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I'm from the US- but want to learn about both.

#### Dan

Admin
Staff have cleared up some posts in this thread. Please keep threads on topic and don't respond to rubbish, just report it, it saves us time. Cheers.

#### Megawatt

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Arms
I'm from the US- but want to learn about both.
So we got something in common good luck
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Staff have cleared up some posts in this thread. Please keep threads on topic and don't respond to rubbish, just report it, it saves us time. Cheers.
Sorry Dan and have a great day

#### Cookie

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We do Megawatt. Thanks

#### Cookie

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Err OK then.

I measured the voltage today and got 122.4 volts slot one to earth, 122.5 volts slot two to earth and 244.5-244.7 volts slot to slot. I can't find or measure any 240 volts to earth. IMO the diagrams are correct.

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#### DPG

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Which diagrams are incorrect?

#### Cookie

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These in post #3:

IMO they are the correct depiction of the systems I'm working with.

#### DPG

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Arms
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Sorry, my mistake - I thought you had put 'incorrect'.
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I think I need to reread this thread. You posted a table from the UK regs I think, but you are asking about the US voltages? Apologies if I'm confused.

#### Cookie

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My apologies. And to be fair I can understand any confusion this may cause as these systems are not used in the UK.
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120/240 and 138/240Y system, not used in the UK, but designing around IEC standards in regards to human and life safety.

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#### davesparks

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Center tapped 240 is 120. Line to line 240 produces 138.

Republic of the Philippines uses this system.

Yes, BS7671 and IEC codes on how they would go about dealing with those two voltages.

I'm well aware line-line 240 is rare in Europe.
BS7671 is primarily intended for use in the UK with our standard voltages.

Using BS7671 in the Philippines with USA standard supplies and voltages is never going to be fully compatible. Its a square peg and round hole situation.

#### Cookie

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BS7671 is primarily intended for use in the UK with our standard voltages.

Using BS7671 in the Philippines with USA standard supplies and voltages is never going to be fully compatible. Its a square peg and round hole situation.

I agree- which, IMO- gives me grounds to question Table 41.1 in BS7671 and IEC60364-4-41. IMO both these codes are 230 volts centric, meaning they give little thought when dealing with systems 150 volts to ground and under. Considering that at one point parts of central Europe were 127/220 (think how schuko sockets came about having no polarity), the IEC/VDE/NFC15-100 may have had incentive to mandate 0.4 seconds disconnection times on 127/220 volts systems since an increase in voltage would not require any re-wiring or recalculating of the circuits once raised to 220 volts to ground.

Both the Phillipines, Mexico, Canada and the US use the NEC or NEC based code- however the NEC has no earth fault loop impedance requirements. Meaning I could legally run 2.08mm2 to a shed 2000 feet away and never trip a breaker during a short circuit. All the earth wires in the shed would be at 60 volts to remote earth. I could plug in something metal, have it sit live at 60 volts to ground, get killed, and absolutely nothing could be done to blame the electrician, authority having jurisdiction or the NFPA. Even better a 277 volt light pole in a parking lot. I could run 3,500 feet of 3.31mm2 to a LED light, have a ground fault in the fixture, energize the pole at 138 volts to ground and have it sit continuously 24/7 live without tripping a breaker where a child could hug or lick the post with their tongue during winter. Child dies, case gets taken to court, lawyers litigate away from the real cause taking advantage of everyone lacking basic electrical theory...

Here are two US sparks having seen the problem in their career:

The thing is people, especially children, get killed all the time in the US from energized pools, fences, light poles, AC units, industrial equipment, doors, ect. The blame gets put on shoddy work or poorly connected earth wires... not the code... Much like in the 70s missing bonding jumpers leading to shocks and electrocutions were blamed on ground rods not driven deep enough in the court of law.

The NFPA's solution is to quietly mandate GFCIs on everything- much like GFCIs took care of all the 2 prong metal framed tools injuring people. I encourage you to read the code making panel's reasoning:

NEC 2020 code new standards in GFCI protection - http://www.p3-inc.com/blog/entry/nec-2020-code-new-standards-in-gfci-protection

Ditto for industrial equipment violating earth fault loop impedance:

Which is why I am so curious about learning BS7671 and establishing a value which can be applied to US systems.

That and the fact the US will eventually one day go to a 230-250 volts utilization voltage. Before (if) 230/400Y becomes the norm 127/220, 133/230 and 138/240Y systems will be common in aiding the conversion process. Its easier to raise a Con Edison 120/208Y network by 10 volts than it is by 110 volts.

But regardless the issue of loop impedance (or lack there of) needs to be addressed first.

#### Cookie

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BS7671 is primarily intended for use in the UK with our standard voltages.

Using BS7671 in the Philippines with USA standard supplies and voltages is never going to be fully compatible. Its a square peg and round hole situation.

I'll ask this: how did loop impedance become a thing in the UK? Was it actual shocking electrics or a theoretical perspective of "what if"?

#### static zap

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I'll ask this: how did loop impedance become a thing in the UK?
I wonder if UK DNO. (Supplies) are over engineered !
Uk electrician then feels obliged to maintain the "Earth" integrity for rest of instal - Need an instrument to confirm the status.

#### Cookie

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If you mean the CPC then I think thats a good idea. But testing earth rods on a TN-C-S and TN-S network I think is not necessary.

#### static zap

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If you mean the CPC then I think thats a good idea. But testing earth rods on a TN-C-S and TN-S network I think is not necessary.
Agreeing with you there ( distinction of TN-C-S / TT ! )

#### Cookie

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Off topic- but what do you prefer/think is better: TN-C-S or TT?

Personally I wish the world had TN-S without the neutral distributed.

#### static zap

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Personally I wish the world had TN-S without the neutral distributed.
My preference would be .. TNC . (PEN)

#### Cookie

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Why TN-C (PEN)- seems backwards.

#### static zap

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Good solid copper connections , without reliance on -expensive -Historic lead - Greyness !
(we have fracking -greed to worry about - too !)
(admittedly -my opinions are more from an interested engineer-Than full spec Spark)

#### Cookie

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Well- in TN-S without the noddle distributed you call always have 4 cores around what ever protects the cable

#### davesparks

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If you mean the CPC then I think thats a good idea. But testing earth rods on a TN-C-S and TN-S network I think is not necessary.
8f you don't test the earth rods then how do you know that they are providing a low enough impedance to earth?

#### davesparks

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I'll ask this: how did loop impedance become a thing in the UK? Was it actual shocking electrics or a theoretical perspective of "what if"?
Loop impedance has always existed, its a property of any electrical supply, its the knowledge, awareness and testing etc which has developed over time.

UK wiring regulations have required testing of, and specified maximum impedances of, earth's for a very long time.
And as the knowledge of the science has improved then so have the requirements, also the ability to test these things has developed allowing them to be tested.

If I recall correctly early versions of the regulations describe test methods which require the electrician to connect a power source of specified capacity and take voltage and current measurements to establish earth impedances.

I've I get chance I'll post some relevant pages from old regs books later.

#### Cookie

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Is a low earth impedance necessary on TN-C-S and TN-S?

#### Cookie

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Thanks- I would be interested. Can't believe the US has not thought of this for over 100 years.

#### davesparks

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Is a low earth impedance necessary on TN-C-S and TN-S?
What exactly did you mean when you asked if it's necessary to test the earth rods on TN systems?
I assumed you meant the earth electrodes at the transformer for the N-E link at source and/or the other TNCS electrodes used along the distributing mains.

#### Cookie

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What exactly did you mean when you asked if it's necessary to test the earth rods on TN systems?
I assumed you meant the earth electrodes at the transformer for the N-E link at source and/or the other TNCS electrodes used along the distributing mains.

Do you have earth rods at the service on a TN-C-S or TN-S system?

#### davesparks

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Do you have earth rods at the service on a TN-C-S or TN-S system?
No, the regulations dont require this.

#### Cookie

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No, the regulations dont require this.

Can I ask why? NEC requires that the service has grounding electrodes. So it is a surprise to hear this.

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