Ground fault trip device will be disabled. 3

[Harry choi]

This is choi from Korea. I’m electrical engineer.
Our client want to eliminate ground fault function in low voltage zone.

TR : 6.6/0.38kV
TR 2nd 3 phase short circuit current : 52kA
TR 2nd Line to ground fault current : 51kA calculated by etap.
Grounding system : TN-S

——
ACB and MCCB have trip device with long time, short time, and ground function.

I believe that there is difference between 3phase fault and ground fault when it comes to equivalent circuit.

But our client claims that high ground fault current can be protected by ACB and MCCB’s phase protection.
It seems to be reasonable because CB will operate when high current of only 1 phase.

I don’t want to eliminate ground fault function. So I’m looking for IEEE or IEC stanrd.

Is there any reason to maintain OCGR function?

 

[waross]

Is there any reason to maintain OCGR function?

Unintended consequences.
Your ground fault protection may be mandated by:
Electrical codes.
Worker safety codes.
Damage repair and replacement insurance.
Downtime insurance.
If the ground fault protection is integrated in a breaker then eliminating the protection may void warranty and more important the approvals. That takes you back to the previously listed issues.

Why does your client want to eliminate the protection?
If this is to replace a failed GF protected breaker with a non GF protected to save money or delivery time, the question must be asked;
"Why was a GF protected breaker used originally."
The answer to that may answer your current question.

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

 

[che12345]

"..Our client want to eliminate ground fault function in low voltage zone..".
I have the following opinion for your consideration.
1. Your system with 3-ph fault 52kA and 1-ph fault 51kA. It is true that the ACB and MCCB will trip at this high current level.
2. In the LV design the GF is (always suggested set << OC level); without exception in IEEE/NFPA(NEC) and IEC standards.
3. Frequency of GF is very much more frequent than a 3-phase shot. It is very much desirable to have GF protection (usually set < 10% of the rated current). This is to ensure that a low GF (<< rated current) is detected and trip off with some (say < 0.5s) time delay.
WARNING If GF protection is deleted, the GF fault current must > the ACB/MCCB OC level to trip out the system.
4. If for any reason that your client wishes NOT to trip out on GF, change the system to IT (3ph 3W); instead of TN-S. This is the marine practice, where 1ph to Ground does NOT cause any tripping of the system. Attention: IT system needs insulation monitoring etc...
Che Kuan Yau (Singapore)

 

[Harry choi]

Thanks for your help.

Our client believes that only high ground fault occurs in TN-S system. And also there is no circumstance to not be protected by OC relay.
It sounds like good plan.

But I wonder that ZCT burden when OC relay fails to open circuit breaker as well as cable short time capacity.
In addition, as we know it differs from 3 phase and L-G fault equivalent circuit such as Z0, Z1, Z2 impedance. In this circumstance, we have to coordinate over current TCC curve under the ground fault system.

But When it comes to client theory, OC relay can always protect ground fault.

In my point of view, most important thing is that ‘ground fault current is too low enough to not protect by OC relay’.

 

[che12345]

"...#1. But When it comes to client theory, OC relay can always protect ground fault. #2. In my point of view, most important thing is that ‘ground fault current is too low enough to not protect by OC relay’..".
I have the following opinion for your consideration.
1. No. Do NOT depend on the OC of ACB/MCCB to trip on low GFs. GF setting is always << OC of ACB/MCCM. Use 4 Protection class CT for OC and a separate ZCT for GF is a very good practice. The additional cost is minima.
2. I agreed with your point.
3. The single ZCT will be able to detect low GF current. Add a small delay say 0.5s would be useful. GF tripping time < the OC hi-set time delay. This is to ensure that in case the GF over-shoot the OC setting, there is coordination between GF (to trip first) instead of the OC protection.
Che Kuan Yau (Singapore)

 

[RRaghunath]

Theoretically Ground fault current level can be as high as (or even higher if there is generation close by) the three phase fault current level. But, ground fault currents in the field are always quite low due to the ground loop impedance (that includes impedance at the point of fault).
Even one Ohm Ground loop impedance can mean a ground fault current of 380A or lower (in case of 380V system). It is unlikely Phase Overcurrent protection can be effective in such a case. This is the reason, a distinct, dedicated ground fault protection with sensitive pick up is always part of the protection scheme in electrical systems.
Phase Overcurrent protection pickup cannot be lower than the load current of the subject feeder or incomer whereas, the pickup threshold of ground fault protection can be sensitive. Typically, the ground fault protection pick up is set 10% of the feeder rated current.

R Raghunath

 

[Harry choi]

Thanks for your help. I appreciate.

I'm aware of that just 1 ohm make low level ground fault current. But in our TN-S system, ground fault loop consist of conductor and PE (Protective Conductor).
So neutal resistance earthing to ground will be removed in ground fault circuit. (This is our client's opinion.)

I know that Z0, Z1, Z2 impedance in zero phase circuit is not determine exactly. But we can calculate it with electrical program such as etap, we know that ground fault current is high enough as much as 3phase short circuit current. In this situation, there is some reason we are able to eliminate GF protection.

p.s Client's sole purpose is not saving the cost. They want to avoid MCCB trip when unbalance and 1-phase open circumstance.

 

[waross]

They want to avoid MCCB trip when unbalance and 1-phase open circumstance.

Well, the rest of the story;
A properly implemented GF trip should not trip on "unbalance and 1-phase open" conditions.
Be aware that "1-phase open" may cause equipment damage, and eliminate it at your peril.
If there are any codes or requirements for GF trips than the unbalance trips should be replaced with properly designed GF trips.
There are always exceptions but in most situations.
I would not be concerned with eliminating unbalance trips.
I would be careful of unintended consequences when eliminating open phase protection.
I would resist eliminating true GF trips.


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

 

[RRaghunath]

"...trip when unbalance and 1-phase open circumstance" - it is a misunderstanding to think the ground fault protection can maloperate on unbalanced load or on 1-phase open condition.
Neither of these can result in current flowing between neutral and earth.
In case of 4-pole breakers, the ground fault protection CT surrounds all the 4-poles, thus load unbalance is taken care.
In case of single phasing (1-phase open), the current circulates between the two remaining phases (at a phase angle difference of 180 degrees) and there won't be any current in the neutral.
Being TN-S system, I suppose you have neutral busbar in the switchgear and the incomer circuit breakers are 4-pole type.
If you are using 3-pole breakers by any chance and taking neutral busbar directly (bypassing the breaker), you may like to add a CT in the neutral circuit and wire in to the ground fault protection scheme. This is also a standard arrangement and your switchgear vendor should be able to help (also provide the CT in the neutral circuit).
If you don't want any of the above and also don't want the MCCB protection to maloperate for ground faults, choose the MCCB setting higher than the expected load unbalance. In this scenario, the ground fault protection at the incomer level be provided by the standby earth fault protection connected to transformer neutral CT.

R Raghunath

 

[edison123]

A lps Raghunath for great explanation.

Muthu

http://www.edison.co.in

 

[Harry choi]

I appreciate for your help.

I summarized below step.
1. In fact, OCR can trip circuit breaker in high level ground fault.
('Ground fault current of a sufficient magnitude may be detected by phase-overcurrent devices.' - IEEE 242)

2. We need to calculate ground fault current in TN-S system. We have to put PE conductor's impedance, cable length in electrical calculate program for making suitable zero sequance loop.

3. In TN-S system, only high level ground fault exist in case of feeder ground fault.
(Ground fault protection is not protection from high level ground faults. - article in Cooper Bussmann)
(

https://www.eaton.com/content/dam/e...ground-fault-protection-requirements-gfp.pdf)

4. Arc ground fault can make unexpected situation.
(I'm not aware of arc ground fault magnitude. In my point of view, it could be the reason that ground fault not disable.)

I'm looking for many standards and reference.
I also though many things such as CT burden, cable short time capacity, principle of LV trip relay, TN-S system, and arc ground fault.
I realized that there are many step to develop my engineering skill.

Thanks for your help. I'm always happy to take constructive Criticism.