Hypothetical - exceeding switchboard ratings
Hypothetical - exceeding switchboard ratings
(OP)
Within my employer there has been some recent discussions regarding ratings of some of our equipment, and instances whereby that rating may be exceeded. To this end, a couple of considerations occurred to me, and I'm curious to hear others' views on it.
Situation: Board previously certified as adequate for service (by engineer of record), despite the fact that under certain conditions, should a fault occur, board rating would be exceeded.
A project is raised to replace some equipment connected to the board with other new equipment of slightly higher rating. EOR states during detailed design that board is acceptable with new equipment. Commissioning team raise an issue that during commissioning, number of connected units must be limited in order to not exceed switchboard rating whilst team is in room containing switchboard. EOR agrees to limitation of connected devices and administrative management of system to ensure that rating is not exceeded whilst personnel are present.
Connected equipment consists of LV generators; as background.
So, the hypothetical is thus: EOR approves a piece of equipment as suitable for service with full knowledge of possible exceeding of ratings under certain (admittedly rare) conditions. EOR agrees to and proceeds with mitigation strategies for use whilst personnel are present around equipment. Is the EOR's consideration appropriate, or should the EOR either have:
a) Presented consideration that the exceeding of the ratings not pose any risk to personnel working in the area
b) Retracted the certification that the equipment is adequate for service, given the implication that further measures are required to ensure personnel safety for the equipment.
There is no consideration of replacement of switchboard in the immediate future.
Before anyone asks, this is not an ethics assignment. Any thoughts on whether the EOR is acting in an appropriate manner?
Situation: Board previously certified as adequate for service (by engineer of record), despite the fact that under certain conditions, should a fault occur, board rating would be exceeded.
A project is raised to replace some equipment connected to the board with other new equipment of slightly higher rating. EOR states during detailed design that board is acceptable with new equipment. Commissioning team raise an issue that during commissioning, number of connected units must be limited in order to not exceed switchboard rating whilst team is in room containing switchboard. EOR agrees to limitation of connected devices and administrative management of system to ensure that rating is not exceeded whilst personnel are present.
Connected equipment consists of LV generators; as background.
So, the hypothetical is thus: EOR approves a piece of equipment as suitable for service with full knowledge of possible exceeding of ratings under certain (admittedly rare) conditions. EOR agrees to and proceeds with mitigation strategies for use whilst personnel are present around equipment. Is the EOR's consideration appropriate, or should the EOR either have:
a) Presented consideration that the exceeding of the ratings not pose any risk to personnel working in the area
b) Retracted the certification that the equipment is adequate for service, given the implication that further measures are required to ensure personnel safety for the equipment.
There is no consideration of replacement of switchboard in the immediate future.
Before anyone asks, this is not an ethics assignment. Any thoughts on whether the EOR is acting in an appropriate manner?





RE: Hypothetical - exceeding switchboard ratings
I think the switchboard should not have its rating exceeded in any circumstance.
I am thinking firstly all the warrenty on the equiptment is null and void and what happens if the circuit breaker cannot open to clear the excessive rating?
desertfox
RE: Hypothetical - exceeding switchboard ratings
Let's consider the case where ratings are exceeded when personnel are not present. Does this not weaken the protection equipment with every over-rated hit? What's to say the equipment's rating has now not dropped below what is supposed to be the upper limit when personnel are present? You have now put personnel in a potentially harmful position without their knowledge.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Hypothetical - exceeding switchboard ratings
RE: Hypothetical - exceeding switchboard ratings
The rating of the switchboard that is exceeded is the fault withstand, and its only likely to be exceeded for a short (far less than one second) time when all units are connected to the board. Board is not new, rather its somewhat aged.
RE: Hypothetical - exceeding switchboard ratings
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Hypothetical - exceeding switchboard ratings
I was more curious of the legal standpoint should an EOR state that something is fit for purpose without any safety risks or further risk management measures being required, and then agree to having risk management measures (such as not connecting all units at once) being in place to safeguard personnel.
To my way of thinking, should an issue happen, the EOR would not be in an easily defensible position as they had acknowledged the necessity of the risk mitigation measures.
The details around the discussion point were part of the example, this is, incidentally, the same sort of issue that gets discussed here regarding exceeding fault ratings for the duration of paralleling sources. However, I didn't want to steer the discussion into whether its appropriate for such situations, rather to discuss the resultant liability from the decisions made in the process.
RE: Hypothetical - exceeding switchboard ratings
For a better answer, I'd check with the owner's property insurance carrier for a statement of liability for potential damage to the equipment, and also with the owner's liability insurance carrier for a statement of liability for potential injury to a person or persons.
Best to you,
Goober Dave
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RE: Hypothetical - exceeding switchboard ratings
Since the standards committee that defined the rating conditions cannot be expected to anticipate all possible applications of the board the "Rating" is almost certainly conservative.
So it is possible that an engineer with knowledge of the relevant standard and of the specific application could perform a technically valid assessment and determine that it's ok. It's even possible that the relevant standard codifies how this should be done.
So did that happen in this case, or is he just guessing?
RE: Hypothetical - exceeding switchboard ratings
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Hypothetical - exceeding switchboard ratings
I also agree with MacGyver's consideration about how close does one stop before the limit, otherwise, what is the point of having limits if they're acceptable to break under operating circumstances.
I will state, though, that the board was type tested to the relevant fault withstand for the busbar assembly, though the original certificate showing test results is not available. Rating given was for fault current and duration, and there is reliance on the generator's ability (or inability, perhaps...) to sustain a fault for a certain duration as justification.
RE: Hypothetical - exceeding switchboard ratings
That doesn't describe assuming. It describes engineering.
Since we have a bit more knowledge of the specifics now it appears to be a heat issue - current, voltage and time.
So it seems possible that a valid assessment could be done.
We don't know if it was in this hypothetical case.
There is still potential liability, and even if there is no actual liability the potential costs associated with litigation could be significant.
RE: Hypothetical - exceeding switchboard ratings
RE: Hypothetical - exceeding switchboard ratings
The EOR should state there is no risk is that is clear in his mind.
Aditionally if we are going to break the rules for rating of the switchboards then there seems little point in using them to test the switchboards in line with them.
If the board is rated for a certain fault current for one or three seconds then thats the limit, can anyone predict what would happen after a fault continue's for 1.5 or 3.5 seconds?
RE: Hypothetical - exceeding switchboard ratings
You might want to check that these aren't standard distribution class breakers which will probably not be designed for the higher assymetric peak current and the long time to first current zero which typically occurs in generator applications. Distribution breakers aren't designed for such severe breaking conditions and can fail when called upon to break a heavy fault. If you're already at the limit of the equipment's capability then this might well be too much for it to handle.
Of course distribution class gear employed in a generator application could be the origin of the EOR's concern...
RE: Hypothetical - exceeding switchboard ratings
To me there are really only two options, there is *no* risk and mitigation measures are not needed, or there is a risk and mitigation measures are required.
Of course, in all of this, there could well be others pushing for risk mitigation that isn't necessary but is placatory instead, but I've assumed this not to be the case.
ScottyUK, you'd be right, but these are LV units, possibly not even large enough to be considered as backup plant for the scale of gear you're normally used to. Breakers in this case are actually rated to withstand opening of the full fault current, including exceeding the switchboard rating (actually they're rated a bit higher than that again, but still). Its certainly a point that bears repeating however.
RE: Hypothetical - exceeding switchboard ratings
Yes.
It's a physical process that follows well understood rules.
So yes, an engineer can predict what will happen under an identified set of conditions.
In fact that's exactly what the "Rating" is - a prediction that under a set of conditions nothing bad will happen.
Those saying "just obey the rating" are effectively saying that engineers are not necessary, as everything becomes just a set of IKEA assembly instructions.
RE: Hypothetical - exceeding switchboard ratings
The rating is almost always proven by actual tests, because it is difficult to predict the complex interaction of the bars, supports, braces, droppers, and so on. Not impossible given sufficient computing horsepower, but only the largest switchgear builders have that capability. Factor in that utilities are very conservative organisations, and when your biggest client base says "We want a real test, not a computer model..." then real tests are what takes place.
RE: Hypothetical - exceeding switchboard ratings
Not because the MCC's are overloaded, but because they "might" blow up evenunder average conditions. Here, a known condition and a apparently a known overload was apparently evaluated by the EOR and accepted, but added precautions were deemed prudent.
That doesn't make the decision right, or the acceptance of the condition prudent, but it is what happened. On the other hand, I have seen broom handles used to support 40 year old cable bundles as well. Doesn't make that right either.
RE: Hypothetical - exceeding switchboard ratings
RE: Hypothetical - exceeding switchboard ratings
http://www.trane.com/commercial/library/vol273/wit...
It clearly states conditions to be met, it doesn't say that it's alright to exceed the rating if it did then what's the point of designing and testing to a given fault rating, here is an extract:-
Overcurrent protective devices (such as fuses and circuit breakers) should be selected to ensure that the short-circuit withstand rating of the system components will NOT BE EXCEEDED should a short circuit or high-level ground fault occur.
System components include wire, bus structures, switching, protection and disconnect devices, distribution equipment, etc., all of which have limited short-circuit ratings and would be DAMAGED or DESTROYED if these SHORT-CIRCUIT RATINGS are EXCEEDED. Merely providing overcurrent protective devices with sufficient interrupting ratings will not ensure adequate short-circuit protection for the system components. When the available short-circuit current exceeds the withstand rating of an electrical component, the overcurrent protective device must limit the let-through energy to within the rating of that electrical component.
RE: Hypothetical - exceeding switchboard ratings
djs, the reasons for the lack of clarity presented around the prospective fault current is that this location is powered by diesel generators, there is no incoming feeder or single breaker for a utility feed.
Thus (and this is what ScottyUK would have considered when he pointed out breaker ratings) the prospective fault depends on number of machines connected and so on. In our discussion case, when all units are online, the peak fault exceeds the board rating as stated in the test documentation (type test certificate), for a period in the range of around a 1/4 of a second. After this point its expected (as per manufacturer's documentation and typical tests) that the fault decays to far less than the board rating. This is entirely due to the size of the unit, much larger machines behave in the same manner, though the numbers differ.
This is all to do with the technical details of whether or not the rating has been exceeded though, rather than the original question posed.
RE: Hypothetical - exceeding switchboard ratings
How many samples are tested?
Enough to cover every possible permutation of complex interactions? Of course not.
Is manufacturing variation considered correctly? Are all possible material defects considered correctly? Even probably defects?
A "worst case" sample? Probably not, because has you correctly state, it's difficult to determine what is really "worst case".
The truth is that most tests are woefully inadequate to do anything remotely useful other than make people that don't understand this feel good.
RE: Hypothetical - exceeding switchboard ratings