Difference between CB and Switchgear rating?

[Prot123]

Hi,
I'm working on a project to determine a suitable protection for a MV(11kV) metal clad switchgear. Being an old system I was able to track the short circuit rating of the CBs, however no luck with the rating of the busbar and switchgear. I found that people confuse with CB and switchgear and assume same rating for both. My understanding is they different and must have different rating. What do you think? and also what is the best approach forward in this kind of situation. I don't think its practical to assume same rating of CBs and the busbar. Bit confused with what to do. Can anyone shed some light. Cheers

 

[waross]

Compare the short circuit rating of the bus bars and of the circuit breakers.
The lowest rated component in the switch gear will determine the maximum rating of the switch gear.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Prot123]

Thanks Bill. That's the whole issue. SC Rating of busbar is not available. Could only find the SC rating of CB.

 

[LionelHutz]

The people saying it's the same probably believe the manufacturer would have matched the gear rating with the breaker rating. It's typically done that way these days. A 40kA breaker would be installed in 40kA gear. But, who knows with older gear.

However, given no option you can either assume a low value you believe is safe, use the breaker rating if you believe is safe or replace the gear. You could compare bus bar size and bracing to other gear with the same rating for clues.

 

[Prot123]

Thanks LionelHultz. One thing, if a short circuit rating of switch gear is "20kA for 3sec" does it imply that the switch gear is safe if the fault(which is less than 20kA) is cleared within this time. without dedicated bus protection, the upstream has to clear the fault which is around 1sec. So what happens to the switchgear given that the bus and VCB are maintained regularly.

 

[davidbeach]

For breakers it is an interrupting rating, for the gear it is a withstand rating for through faults. There's no survivability rating for faults in the gear. If you have a bus fault it doesn't matter whether you had bus protection or not, the gear is damaged. That 20kA for 3 sec rating says that a 20kA fault downstream somewhere, fed through this gear, will not cause the gear to fail. Part of that is the magnetic forces associated with the fault current and part of it is the thermal impact.

 

[Prot123]

Just on this David, I thought busbars too have short circuit withstand rating. Doesn't that mean a busbar should be able to withstand a bus fault if it falls within that rating. "20KA for 3sec" doesn't mention anything about the location of the fault. Also for a busbar it depend on how well the arc is contained. Not sure if my understanding is correct.

 

[waross]

The mechanical forces include the forces resulting from the DC offset of the first few cycles at a specified X/R ratio of the supply. The thermal impact increases with time, hence the 3 second limit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[aussiejohn2]

G'day Prot123,
David has it right. An equivalent question is: what is the fault rating of a breaker that has had a mechanical failure? Not many CBs have such a rating! If a fault on the bars themselves is bolted then it may be reasonable to expect the bar to withstand it, both the mechanical forces and the thermal effects (from Joule heating in the resistance of the bars). However if the fault is arcing, it liberates far more heat (and in a far smaller area) than the bar R.I^2. The arc will motor away from the source to the end of the bars, where it will stay while it vaporises the copper and steel. No bar can withstand this. A very fast acting protection (pyrotechnic fault limiter or short-circuit-er) may save the board but that does not really represent a withstand rating for the board in the conventional sense. The withstand rating is for through faults.
Further to Lionel's comments on comparing with other gear, the ratings (both thermal and magnetic) are in principle calculable given details of the bars and their mounts. However not everyone is confident in those calculations (hence why people ask for type tests) and especially if manufacturer is unavailable it may be difficult to find a 3rd party who will stand by them - and insulator manufacturer may no longer be available to give you capabilities of the mounts.
JD.

 

[krisys]

If this is an old installation, check what is the available short circuit level at the switchgear location. If the SC level is well below the breaker SC rating (say 75% of the breaker rating), and if there is no big generator connected to the switchgear or near this switchgear, then you not have any issue. But this is a common sense engineering.

 

[Prot123]

Hi Krisys,
Thanks for the reply.
The whole issue is I don't have the rating for the switchgear. Let me explain in a clear way.
The switchgear is 44 years old and the OCB was retrofitted with a VCB decade back. The rating of VCB is available(25kA, 3sec) but nothing for switchgear and without a dedicated bus protection any fault at the switchgear is cleared slow, more than a 1 sec. From the VCB's rating looks like it can handle the short circuit without much issue, however, nothing can be said of the switchgear. The only comforting fact is its only VCB which has to break/make the fault , whereas, bus bar will just carry the fault current.
Also I didn't get why generator location relevant to switchgear matters as I have short circuit current and X/R ratio at the switchgear available.
Hope this clears things out.

 

[veritas]

The VCB's 25kA for 3s is a throughfault withstand rating as was mentioned earlier. Note it is 25kA rms for a symmetrical current. The VCB will also have a dynamic withstand rating which is based on an instantaneous peak current, Ip. Ip is normally a multipler of the withstand rating (theoretical maximum is 2*sqrt(2)).

What the factor is in your case will depend on the standard to which the VCB was manufactured. The standard will base the factor on a certain X/R ratio. If your actual X/R is higher then you will need to take this into account when determining the suitability of the breaker for the installation. The location of a generator could very well influence the X/R ratio - so yes, the generator location relevant to the switchboard does matter!

A few years ago, I dealt with a similar issue. 11kV swbd around 4 decades old. No nameplate or markings or anything. SWBD was pressure tested and found to make the grade - only just! As the protection was to be upgraded, we decided to include a blocking scheme to bring down the busfault clearance times by a factor of around 5!

A a swbd ages, it becomes a question more of when, rather than if there will be insulation failure somewhere.

 

[Prot123]

Yes I understand the generator location relevant to switchboard matters, but as we have X/R ratio and sub transient short circuit current do we worry about the generator location. The influence of dc transient decaying should be covered in the X/R ratio, isn't it?

 

[waross]

Cable impedance between the generator and the switchgear?

Bill
--------------------
"Why not the best?"
Jimmy Carter