For years we have been specifying Double or Reinforced insulation between Primary & Ground and Primary & Secondary, as described in the Rich's 3rd to the last paragraph. This specification alerts the engineer to look for off-the-shelf sources that meet this requirement, and it also alerts the source what the requirement is in case of custom supplies.
Tania Grant, [email protected] <[email protected]> Lucent Technologies, Communications Applications Group ---------- From: Rich Nute [SMTP:[email protected]] Sent: Tuesday, September 28, 1999 1:51 PM To: [email protected] Cc: [email protected]; [email protected]; [email protected] Subject: Re: Re: EN 60 950 and Denmark Hi Peter: > This is the exact deviation which I was referring to. One of my > clients states that a distributor wishes the product to be safe for > those establishments where earthing cannot be relied upon. Does > this mean to provide double or reinforced insulation from hazardous > live parts to earth? This may be a pain killer if the power supply is > Apporved for Class I. Not necessarily. Today's designs of SMPS are typically double or reinforced insulated. Even if certified as Class I, you have the option of evaluating it for double- or reinforced-insulation and declaring it as complying with double- or reinforced-insulation for use in Denmark (and other Nordic countries where grounded outlets may not be present in the electrical installation). There are two insulations that must be considered: primary-to-secondary primary-to-ground 1. Primary-to-secondary. In a typical SMPS, there are three such insulations that must be considered: transformer opto-isolator PWB spacings Typical SMPS transformers are double-insulated. It is very inefficient for a SMPS transformer to employ a grounded shield. If the certifier did a good job, he will have ascertained that the solid insulation, the through-air insulation, and the surface insulations meet the respective double-insulation requirements. In my experience, most SMPS transformers do meet these requirements. Opto-isolators are almost always rated as double- or reinforced-insulation. Most PWB spacings are those for double-insulation. If not, the secondary circuit and conductors must be grounded as if they were protective conductors. So, I believe you will find that most PWB spacings are sufficient for double-insulation. 2. Primary-to-ground. In a typical SMPS, there are two insulations that must be considered: PWB assembly and the metal chassis PWB spacings across Y capacitors and similar spacings Here is where you are likely to find spacings only sufficient for Class I. Most power supply vendors will likely balk if you should require Class II construction between the circuits and ground. But, you can easily get around MOST of the spacings by interposing a solid insulating sheet between the circuits and the grounded parts (e.g., chassis). You can get around the Y capacitor spacings by asking for double-insulated spacings on the PWB conductors. Its really not too difficult. Its really easy if you spec your power supply as double-insulated primary-to-ground and primary- to-secondary. Almost zero cost. By the way, we find that the weakest insulation in such designs is the Y-cap lead spacing on the PWB. We experience arcs in the range of 4-5 kV, well above the required 3 kV! We do this on all of our products. We KNOW our products will be used in situations without ground. So, we require our power supply vendors to provide double-insulation as well as grounding. Best regards, Rich --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
