Thermal effect of frequency in SWBD Temperature Rise Test (IEC)

[eljas]

Dear colleagues, I hope someone can explain following statement in IEC 61439-1 10.10.3.1 General:
<Quote>
In absence of test at 60 Hz for currents above 800 A, the rated current at 60 Hz shall be reduced to 95% of that at 50 Hz.
<Unquote>

The Switchboards for Temperature Rise Test are rated for 450VAC/3ph/60 Hz.
Rated Current for Main busbars is 5000 A.
Test frequency is 50Hz.

Q1: By quickly thinking I would expect that the thermal effect would be higher in case of higher frequency (eddy currents, core losses etc.). That seems not be the case.

Q2: Formula for calculating heat effect on bus-bars.

Any comments on the issue above is well appreciated.

 

[ScottyUK]

Q1 - I think you are mis-interpreting the wording of the standard. It is saying 'For a given busbar design, the rating at 60Hz shall be 95% of the rating at 50Hz unless a test has been carried out at 60Hz to prove that the rating is greater than 95% of the rating at 50Hz.'. You have correctly identified the reasons.

Q2 - Read Pub 22 from the Copper Development Association .

 

[eljas]

Dear ScottyUK,

Q1 - Thank's for your interpretation, which seems to be in line with my question. I am now in position to try explain this to the SWBD Maker (non english) prior to the coming testing. I think it may be easier to explain it additionally by referring to for e.g. certain thermodynamic laws behind the phenomen. Unfortunately the text in IEC leaves too much room for interpret it one way or another, and I 'd like to ensure that the SWBD's will be tested properly for maximum temperature rise.

Q2 - Excellent link - appreciated.