Hypothetical - exceeding switchboard ratings

[FreddyNurk]

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?

 

[desertfox]

Hi

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

 

[MacGyverS2000]

Your field may be different, but I can't honestly understand a situation where the EOR would be acting appropriately.

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

 

[djs]

I am surprised that the LAHJ let this pass. Or if they did not notice it, perhaps it should be pointed out to them.

 

[FreddyNurk]

Interesting comments, and I appreciate the feedback.

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.

 

[MacGyverS2000]

It doesn't seem a bit odd to you saying "It only exceeds the rating for a short period of time"? If it exceeds the rating, you've entered the realm where damage can be done, as determined by the manufacturer after (hopefully) running many tests and plugging in the numbers.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[FreddyNurk]

MacGyver, only in repeating it. Despite possible suspicions to the contrary, I'm not the EOR.

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.

 

[DRWeig]

I'm sure that the EOR is taking on some liability with the knowledge that he is doing so.

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

Haven't see the forum policies? Do so now: Forum Policies

 

[MintJulep]

The "Rating" is almost certainly based on one or more sets of defined conditions.

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?

 

[MacGyverS2000]

It just seems to me that assuming a max rating was arrived at by being conservative is a huge assumption that should never be made... how many conservative assumptions were made up the line, to the point where you get to the end of the line and that "conservative" value has all but disappeared.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[FreddyNurk]

I'd only be speculating as to the methodology used by the EOR, after all, this was presented as a hypothetical even though its clearly based around certain circumstances.

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.

 

[MintJulep]

an engineer with knowledge of the relevant standard and of the specific application could perform a technically valid assessment

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.

 

[JStephen]

It sounds to me like the answer could be somewhat variable, depending on how much confidence the engineer had that those procedures would actually be followed.

 

[desertfox]

Seems odd to me that the EOR states the risk is acceptable and then agree's to minimize any risk to personnel by limiting the number of circuits live while people are around.
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?

 

[ScottyUK]

Can I throw in a further complication that generator duty puts some much heavier demands on switchgear than would be found in a normal distribution system?

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...

 

[FreddyNurk]

desertfox, that was exactly the thought that prompted me to start the thread. This struck me as quite awkward and as such I was keen to ascertain whether others thought the same.

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.

 

[MintJulep]

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?

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.

 

[ScottyUK]

"In fact that's exactly what the "Rating" is - a prediction that under a set of conditions nothing bad will happen."

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.

 

[racookpe1978]

But, it is prudent and reasonable and expected in fact, to protect workers and operators even when ALL of the equipment in a high voltage control or breaker room is energized. You need permission to enter, you don't allow untrained people to enter, you don't use it as a workroom or utility closet. Nobody enters the HV motor controller room unless absolutely needed.

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.

 

[djs]

If this product was approved for use in the US, then testing is mandatory. In Europe one can get certification by design and calculation. In any case depending on the age of the panelboard in question, the actual withstand maybe significantly higher. In ages past, cost was not such an overriding concern, and many products were overdesigned to ensure that it passed testing. It also appears from the OP post that the panelboard does not have a single main feeder breaker. If it did then the available fault current to the panelboard would be known.