Converting TT to TNS earthing system to avoid untimely RCD tripping

[AntonioPerez]

Hi all,

An industrial facility owner who suffers from frecuent untimely RCDs tripping in his facility was adviced to convert the existing TT earthing system into TNS. We've been asked to quote the engineering project for that. I've never seen this done before but I guess it is not quite difficult to get the exposed conductive parts connected to neutral earthing. This wouldn't solve the problem so RCDs should be eliminated. Circuit breakers tripping time should be checked to assure that no dangerous voltage happens during an indirect contact. PE conductor section should also be checked.

What do you think? Am I missing something? Have you found this problem before? Do yo find any other solutions?

 

[che12345]

"....#1.An industrial facility ...suffers from frecuent untimely RCDs tripping in his facility was adviced to convert the existing TT earthing system into TNS. ...#2 I guess ...to get the exposed conductive parts connected to neutral earthing....#3 This wouldn't solve the problem so RCDs should be eliminated....#4 Circuit breakers tripping time should be checked to assure that no dangerous voltage happens during an indirect contact. PE conductor section should also be checked".
I have the following opinion for your consideration.
1. I do NOT think that by converting the existing TT to TN-S would solve the problem. See 3 below.
2. Ref IEC 60364-4-41 for TN-S, the conductive part is connected to the PE (green/yellow) conductor to the source Neutral. Only the source Neutral is earthed (RB). Where a TT has a (RA) at the installation and a separate earthing (RB) at the source Neutral.
3. Do you mean "this [would] solve the problem so RCDs should be eliminated"? No. RCD of 30mA is for life saving. Do NOT eliminate them! Other earth-fault or earth leakage protective devices can be [selectively coordinated] by current and time delay etc..
4. Maximum disconnecting time for final circuits < 32A with nominal voltage 230V: TT <0.2s, TN <0.4s. Ref ABB Switchgear Manual.
Che Kuan Yau (Singapore)

 

[waross]

One of the first rules of trouble shooting is to understand the problem before guessing at a solution.
The first step here may be to find out where the source of the RCD trips are.
It may be a heads up that a circuit or machine is on the verge of failure.
I have known many electricians and engineers who would find the source of the excess ground leakage current in short order. (And some who would never find it.)


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[AntonioPerez]

Thanks for your answers.

che12345: I mean than just converting the TT to TN-S wouldn't solve the problem because RCDs will be still untimely tripping anyway. I had guessed that converting TT to TN-S was just a maneuver to be able to eliminate RCDs. Regulations let that circuit breakers be the protection against indirect contacts, as in TN-S this is the same as a phase to ground shortcircuit, so RCDs are not mandatory in TN-S. That's why I think they would want to eliminate RCDs. On the other hand I agree with you that RCDs are an additional protection, life saving, and I wouldn't recommend to eliminate them.

waross: I absolutely agree with you but I think my client had an adviser engineer who studied the problem and recommended this solution.

 

[RRaghunath]

In case of TT system, the earth loop impedance can be quite high, restricting the available magnitude of current during earth faults. This is the reason sensitive earth fault protection devices such as RCDs are mandated in all the feeders.
In case of TN-S system, RCDs / RCCBs are necessary only where protection against electrocution is necessary as well as where Fire protection due to leakage current is necessary.
RCCBs with 30mA sensitivity provide protection against electrocution whereas, 300mA sensitivity can provide protection against only fire.

R Raghunath