Hello R. Keller and Heber: This answer assumes that the standard of evaluation is IEC 950. We manufacture many transformers that comply with this standard.
There would certainly be creepage distances, unless the barrier qualifies as a "cemented joint", in which creepage and clearance distances can be shown not to exist. To qualify the barrier as a cemented joint (see 2.9.7 and 2.9.6). If the part does not qualify as having a barrier with no creepage and clearance distances as outlined in clauses 2.9.6 and 2.9.7, then the internal transformer may qualify as Pollution degree I. Under this condition, CTI ratings do not matter, (see table 6), and the creepage and clearance distances would be equivalent. Some other standards do not seem to specify the qualification tests for a "cemented joint" as specifically as IEC-950, but I would recommend performing some type of thermal cycling test if taking this approach. If neither of the above apply to your case, you must take the worst case CIT rating of the two materials with common surfaces. If you have no rating, you must assume material group III. Hope this helps. Mel Pedersen Midcom, Inc. Homologations Engineer Phone: (605) 882-8535 [email protected] Fax: (605) 886-6752 ---------- From: Farnsworth, Heber[SMTP:[email protected]] Sent: Friday, April 03, 1998 9:55 AM To: '[email protected]'; [email protected] Subject: RE: Transformer potting + CTI spacer tape My first thought is that, where you have potting you do not have creepage, hence CTI is not involved. However, on a related topic, I've had problems with vacuum-impregnated transformers. It seems that, in drawing a partial vacuum there's often a little air left. Then, even with pressure impregnating, the epoxy doesn't penetrate fully: it has higher viscosity than air, and tape will sometimes pull up, "burping" out the air, then reseal under positive pressure. The result is that pockets of partial vacuum are left in inner windings, and the transformer breaks down at voltages BELOW it's unpotted level! Whenever possible, I recommend the use of split or multi-part bobbins which assure spacings without relying on manufacturing skill. Good, safe transformers are deceptively hard to design, build and verify, especially when coupling requirements prevent the use of such bobbins. __________________ Heber Farnsworth Physio-Control, Seattle, USA > -----Original Message----- > From: [email protected] [SMTP:[email protected]] > Sent: Thursday, April 02, 1998 11:49 PM > To: [email protected] > Subject: Transformer potting + CTI spacer tape > > > Anybody with any experience how potting is influencing the parameter > and > characteristics of CTI rated spacer tape? > > A spacer tape with CTI rating >600 is used, and the transformer will > finally be potted. > > My anxiety is that the potting liquid is penetrating the spacer tape, > hence > degrading the quality of the spacer tape's glue and CTI capabilities. > > What do you think? > > R. Keller >
