Maximum Voltage across CB Contacts?

[dkle40h]

Hi,
What is the maximum voltage that can develop across the contacts of a three phase circuit breaker? This may seem like a trivial question at first but please give it some thought. Assume the circuit breaker feeds a load from the secondary side of a three phase, 11kV/550V, delta-star connected transformer. What is the maximum voltage that can develop across the contacts of a single phase as the circuit breaker operates to clear a fault condition?
Your comments/discussion will be appreciated.
Regards

 

[musicguy1800]

dkle40h,
Are you saying you are interested in knowing the highest *transient* voltage that the circuit breaker can handle?

 

[dpc]

This is actually a rather complicated question. The maximum depends on the nature of the load or fault and other variables. It can be much higher than the normal circuit voltage.

ANSI C37.010 may provide some insight.

 

[busbar]

It is possible for low-voltage situations that something like ANSI/IEEE C62.41 Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits would offer rough scaling of voltages. [TOC at

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.41-1991_desc.html.

Click on the blue dot.]

 

[jbartos]

Suggestion: The circuit breaker withstands its standard rated impulse withstand voltage. This is correlated and consistent with other equipment, e.g. transformers, buses, etc., Basic Impulse Level (BIL).
Reference:
1. IEEE Standard 141-1993 Red Book:
Table 10-5 Rated Voltages and Insulation Levels for AC Switchgear Assemblies on page 495:
RATED VOLTAGE RMS:
Nominal Voltage RMS: 480*, 600**, 13.8***
Rated Maximum RMS: 508*, 635**, 15.0***
INSULATED LEVELS (kV):
Power Frequency Withstand RMS: 2.2*, 2.2**, 36***
DC Withstand: 3.1*, 3.1**, 50***
Impulse Withstand: -*, -**, 95***
where:
* Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear (in Volts)
** Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear (in Volts)
*** Metal-Clad Switchgear or Metal-Enclosed Interrupter Switchgear (in kV)

 

[dkle40h]

Thank you for the comments and my apologies for the delayed response.

I was referring to the maximum system voltages that can develop across the single phase contacts of a three phase breaker. For a phase to phase short circuit, two sets of contacts are in series so as they open to clear the fault (note they are mechnically connected to open at the same time) the max voltage across each must equal half the line voltage, 275V in this case. For a phase to earth fault the max voltage would be the phase voltage, 317V.

The worst case scenario for the contacts would be a phase to earth fault, and I would assume that switchgear are designed accordingly. That is, a 550V breaker would be designed for a max votlage across the contacts in the region of 317V, and for a specific fault rating.

We have experienced a problem in an IT earthed system. The problem arises when operating the plant on a single phase to earth fault condition, when a second earth fault occurs on a DIFFERENT phase. The two faults form a series circuit, due to the high impedance earth, or rather short circuit between two phases. Since the two breakers are not electrically or mechnically linked (different circuits) and due to design differences, one of the breakers attempts to open first to clear the fault, experiencing a LINE VOTLAGE (550V) across it's contacts (only for the faulted phase). The increase in voltage is associated with a reduction in the circuit breakers fault clearing capacity.

Any comments on the description above?

Regards

 

[stevenal]

You are assuming that mechanically interlocked poles clear simultaneously. The poles actually clear according to current zero crossings. In the case of a three phase fault on a single circuit, the second pole to clear sees full line to line voltage (plus any transient overvoltage). The breakers are designed for this duty.

 

[busbar]

dkle40h, this sounds like a similar discussion in thread238-86146 that dealt with molded-case breaker single-pole interrupting ratings. That thread dealt with it in a ANSI context for ungrounded or high-resistance-grounded low-voltage systems—I don’t know how an IT system compares.