Metal-Enclosed, Medium-Voltage Switchgear : cable earthing switch: ike, tke, ipe values

[obut4]

Hello,

I would like your thoughts on the following situation.

We are about to order an Air-Insulated, Metal-Enclosed, Medium-Voltage Switchgear according to IEC 62271-200.

The switching-device compartments are equipped with withdrawable Vacuum circuit-breaker.
The feeder earthing switch can only be operated with switching device in disconnected Position.

My concern is about the Ike, tke and Ipe values for the earthing switch.

The switchgear data are following :

• Installation: Indoor
• Number of phases: 3
• Busbar system: Single busbar
• Rated voltage: 24.0 kV
• Operating voltage: 16.0 kV
• Rated frequency: 50 Hz
• Rated short-time withstand current: 16 kA
• Rated duration of short-circuit: 1 s
• Rated peak withstand current: 40 kA
• Rated short-circuit breaking current: 16 kA
• Rated short-circuit making current: 40 kA

Earthing switch on the cable connection:

• Earthing switch with short-circuit making capability, Short-circuit current and duration: 25.0 kA / 3s
• Short-circuit current and duration:
  
  • Rated short-time phase-to-earth withstand current (Ike): Ike = 60 A
  • Rated duration of phase-to-earth short-circuit (tke): tke ≤ 24h
  • Rated peak phase-to-earth withstand current (Ipe): Ipe ≤ 150 A

The manufacturer states that this is an additional performance after they tested the single-phase short-circuit withstand.
However, the design of the grounding switch has not changed and still maintains its performance at ik = 25 kA, tk = 3s."




Questions:

1. The system neutral earthing is implemented with a grounding resistance, which limits the phase-to-earth fault current to 738 A in the switchgear.
The value Ike and tke are relatively low.

Suppose the cable earthing switch is closed for maintenance and by mistake one phase is energized. This would be a direct phase to earth fault with the fault current limited to 738 A by the earthing system.
This far above the Ike = 60A et and Ipe = 150A.

Is the earthing switch correctly sized?
Is it safe?

2. Is ik = 25 kA, tk = 3s valid only if the three phases are energized ?

3. The earthing switch looks similar to earthing switch in switchgear from other manufacturer.
I could not find the value Ike and the in the datasheets.
Does it mean that they have better performances or that they didn’t evaluate this parameters?

Thanks for your help

 

[7anoter4]

The standard IEC 62271-200 recommended test are as per IEC std 60694.
In this standard, on 6.6.1 Arrangement of the switchgear and controlgear and of the test circuit, the test may be made three-phase or single-phase.
On a three-pole switchgear and controlgear, the test shall be made on two adjacent poles.
If it was so tested and the test arrangement is noted in the test report then, in my opinion, it will act also in your case .

 

[7anoter4]

If the fault clearing time is less than 3 sec the switch is O.K. Usually, the clearing time is 0.25 sec [including auxiliary relay] if no delay is involved.

 

[RRaghunath]

Hello,

I would like your thoughts on the following situation.

We are about to order an Air-Insulated, Metal-Enclosed, Medium-Voltage Switchgear according to IEC 62271-200.

The switching-device compartments are equipped with withdrawable Vacuum circuit-breaker.
The feeder earthing switch can only be operated with switching device in disconnected Position.

My concern is about the Ike, tke and Ipe values for the earthing switch.

The switchgear data are following :

• Installation: Indoor
• Number of phases: 3
• Busbar system: Single busbar
• Rated voltage: 24.0 kV
• Operating voltage: 16.0 kV
• Rated frequency: 50 Hz
• Rated short-time withstand current: 16 kA
• Rated duration of short-circuit: 1 s
• Rated peak withstand current: 40 kA
• Rated short-circuit breaking current: 16 kA
• Rated short-circuit making current: 40 kA

Earthing switch on the cable connection:

• Earthing switch with short-circuit making capability, Short-circuit current and duration: 25.0 kA / 3s
• Short-circuit current and duration:
  
  • Rated short-time phase-to-earth withstand current (Ike): Ike = 60 A
  • Rated duration of phase-to-earth short-circuit (tke): tke ≤ 24h
  • Rated peak phase-to-earth withstand current (Ipe): Ipe ≤ 150 A

The manufacturer states that this is an additional performance after they tested the single-phase short-circuit withstand.
However, the design of the grounding switch has not changed and still maintains its performance at ik = 25 kA, tk = 3s."


View attachment 700

Questions:

1. The system neutral earthing is implemented with a grounding resistance, which limits the phase-to-earth fault current to 738 A in the switchgear.
The value Ike and tke are relatively low.

Suppose the cable earthing switch is closed for maintenance and by mistake one phase is energized. This would be a direct phase to earth fault with the fault current limited to 738 A by the earthing system.
This far above the Ike = 60A et and Ipe = 150A.

Is the earthing switch correctly sized?
Is it safe?

2. Is ik = 25 kA, tk = 3s valid only if the three phases are energized ?

3. The earthing switch looks similar to earthing switch in switchgear from other manufacturer.
I could not find the value Ike and the in the datasheets.
Does it mean that they have better performances or that they didn’t evaluate this parameters?

Thanks for your help

The Earth switch need not be rated to withstand 738A for a long time as the earth fault protection in switchgear is expected to clear the fault in a matter of couple of seconds.
Short circuit making and carrying capacity of the earth switch is important and in this case, it exceeds the symmetrical fault current rating of the switchgear.
Thus, the earth switch parameters seem to be OK for the switchgear.

 

[obut4]

If the fault clearing time is less than 3 sec the switch is O.K. Usually, the clearing time is 0.25 sec [including auxiliary relay] if no delay is involved.

Dear 7anoter4,

Thank you for your reply.

I have contacted the MV Switchgear manufacturer.
He explained that the design of the cable earthing switch has not changed but they had decided to test the earthing switch for single phase to earth fault.

The result is that the cable earthing switch can withstand: ike = 60A for tke<= 24 h and Ipe(peak) = 150 A.

If I had to design the cable earthing switch, I would have chosen ike = 1000A for 3s as the earth fault current is limited to 738A on the installation.

My first concern is people safety.
In case one phase is energized while the cable earthing switch is closed, the fault current will be 738A which is above Ipe = 150 A.
The manufacturer didn't seems to be worry about it.

Do you have MV switchgear where the value ike, Ipe et tke appear on the nameplate?
Apparently, other manufacturers do not test these values for the cable earthing switch in MV metal enclosed switchgear.

Best regards,

Obut4

 

[obut4]

The Earth switch need not be rated to withstand 738A for a long time as the earth fault protection in switchgear is expected to clear the fault in a matter of couple of seconds.
Short circuit making and carrying capacity of the earth switch is important and in this case, it exceeds the symmetrical fault current rating of the switchgear.
Thus, the earth switch parameters seem to be OK for the switchgear.

Thank you for your reply