1500VDC Switchboard

Question

I am currently working on a project involving a DC charger rated for a 1500VDC, 3200A switchboard. We have a type-tested switchboard rated for a 100kA/1s main busbar, with a peak withstand of 220kA at 690VAC. The new switchboard, which should be rated for 1500VDC, 3200A, also requires a 100kA/1s DC busbar. 
 My question is: Can we still use the same busbar that has already been tested for AC circuits? For now, let's set aside dielectric strength and focus solely on short-circuit withstand strength. 
 this is a clause from Clause &ndash; IEC 61439 2020 - 10.11.5.4 
 &lsquo;&rsquo; An AC short-time current test may be used to verify a rated DC short-time withstand current and a rated DC peak withstand current providing the test with AC has a peak current equal to the rated DC peak withstand current and the RMS value of the short-time current is at least equal to the rated DC short-time withstand current. When substituting a DC test by an AC test, it should be recognized that the thermal and dynamic stresses of the AC test are higher than those of the equivalent DC test. &lsquo;&rsquo; 
 If the AC short-circuit withstand test is more rigorous than the DC test, my understanding is that we can use the same busbar for the DC application as long as we maintain the same clearances and design parameters as the AC-tested switchboard. Is this correct?

Tim Boler · Accepted Answer

What is the required DC peak withstand current of the DC busbar? My understanding is that DC circuits can have very high peak fault levels because of the effects of parasitic capacitance. The evaluation of peak and withstand equivalence go together according to the standard but, yes, in theory the AC rms withstand is equivalent to the DC withstand.

mapj1 · Answer

In terms of thermal current rating and magneto-strictive stresses your assumption is correct - much as with wire conductor sizes for a given current - one can read across from the 50Hz results to the performance at DC. 
 The problem with DC is that the arcs are much harder to extinguish as there is no zero crossing and assumptions about fault clearance times that work well on AC, may not be adequate for DC. So it is not safe to extrapolate discrimination with fuse breaking times or breaker operating data for example. So the fault current may flow for longer, and then the same fault current may be more destructive. Generally things rated for interrupting high currents at DC end up quite a bit heavier than a similar rated the AC counterpart, and if it is unclear, the makers should be consulted. 
 M.

1500VDC Switchboard

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