Tension between neutral and ground 2

[giordanonaval]

Hi!
Does anyone knows what is the maxim allowed tension between ground and the neutral in a TT system??
Thanks in advance
Martin

 

[ScottyUK]

In which country?


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I don't suffer from insanity. I enjoy it...

 

[waross]

What do you mean by a TT system?
respectfully

 

[Skogsgurra]

TT = Terre - Terre. Describing a system where system is grounded at transformer (the first T) and housing separately grounded at load (second T).

It does not say if Neutral or one corner of the system is grounded at transformer (both are allowed).

So the question is not quite clear. You probably mean between load neutral and ground?

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Hi waross,

It's European terminology. TT is an abbreviation for Terra-Terra. The source is grounded to a local earthing electrode or mat. The customer has a similar electrode at the service intake and there is no metallic ground connection between customer and source. Earth loop impedances are typically high and require use of an RCD as the intake switching device.

Other related European terms are:
[ol]
[li]IT (Isolated-Terra) for ungrounded sources with an earth electrode at the customer intake.[/li]
[li]TNC (Terra-Neutral Combined) for a metallic combined earth and neutral conductor connected to an earthing elctrode at the source.[/li]
[li]TNS (Terra Neutral Separate) for a metallic earth conductor, separate from neutral, connected to an earthing elctrode at the source.[/li]
[li]TNC-S (Terra Neutral Combined-Separate) for a combined neutral and earth up to service entrance connected to an earthing elctrode at the source, with separate neutral and earth conductors available at the customer's intake.[/li]
[/ol]

TNC-S is frequently seen in the UK as Protective Multiple Earthing or PME. PME allows the distribution company to effectively earth the neutral conductor throughout its length as well as at the substation earth mat, saving the cost of running a separate earth conductor.

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I don't suffer from insanity. I enjoy it...

 

[giordanonaval]

Thanks to all of you!
I need the maxim allowed tension between the neutral (in the load) and the ground (in the load). Because I live in Argentina, I need the IEC standard values.
A TT system was well explained by "skogsgurra" and "ScottyUK".
So, if anyone knows the IEC Standard were I can find this, I will be very thanksful.
Thanks again and sorry about my english.
Martin

 

[waross]

Thanks skogsgurra and ScottyUK.
We used to see "T-T" lighting transformers. It was an adaptation of the old Scott two phse to three phase connection. It was used with two transformers to transform three phase to three phase. Often 480 V to 120/208 V.
I was pretty sure that it didn't fit this question.
Respectfully

 

[Kiribanda]

giordanonaval,

The relavant IEC standard which specifies the TT/ TNS systems is IEC 60364 series. Since IEC are a set of specifications and not installation codes like NEC/ CEC, IEC 60364 doesnot specify a maximum allowable N-E potential.

Based on IEC 60364 series, IEE Wiring Regulations 16 th Edition (BS 7671) is the installation code in IEC/ BS world. Even in that, nothing has mentioned about the max. alloawble potential between N-E.

But as per the TT topology, the maximum possible potential between N-E should be equal to ZERO.

"Because I live in Argentina, I need the IEC standard values"

What is your electrical installation code in Argentina?

Hope this helps!

 

[giordanonaval]

Thanks kiribanda
By I understand, the maximum potencial in not equal to zero.

"What is your electrical installation code in Argentina?"

If when you say "code" means "standard", In argentina we use IRAM standard that is extracted from the IEC.
Please teel me if this is ok for you
Sorry about my english
Best regards

 

[waross]

The voltage between neutral and ground will be the product of the current and the impedance of the current path. If this is a system with a neutral conductor connected from the source to the service, the total impedance will be only slightly less than the impedance of the neutral conductor from the source to the service (or from the source to the point of the ground fault).
You may use the impedance of the neutral path and the available phase to ground/neutral asymetrical fault current to estimate the maximum neutral to ground voltage. If you feel that it is too high for safety, you may lower it by adding parallel neutral conductors. That is, reducing the neutral impedance.
Note that the voltage from the neutral to the source ground may be much more than the voltage from the neutral to the service ground. In the event of heavy fault currents you may develop "touch and step potentials" in the vicinity of the ground rods. The touch and/or step potentials will not be as severe with ground grids.
Sorry, I can't help you with the code reference as to what the voltage limit is.
respectfully.

 

[Kiribanda]

giordanonaval,

"In Argentina we use IRAM standard that is extracted from the IEC"

So did you check whether there is a specified allowable maximum potential between N-E?