Specifying the Proper Transformer

7/31/2019 Specifying the Proper Transformer

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Specifying the proper Transformer

Signal Transformer Page 1

The int ernational standards governing electronic equipment have specific requir ements for tr ansformers.This exclusive r eport describes these standards and their impl icati ons for designers. It also explains hownot paying close att ent ion t o standards can result in cost ly, embarrassing, and t ime-consuming redesigns.

In today's inter nat ional market place, manufacturers of electronic equipment are faced wit h a bewildering arrayof standards, covering everything f r om paint to c ir cui t board materi al . I f t here was j ust one great global

standard, t he task of meet ing i t s requirement s would at l east be relat ively str aight f orward. Unfort unately,there are numerous standards, from a variety of agencies in the United States, Canada, and Europe, that mustbe accommodated. Each standard t reats i ts subj ect di f ferent ly, complicat i ng mat ter s furt her.

Lurk ing wit hin each of these standards are requirement s that specify how power t r ansform ers wi l l be bui l t andimplem ented. I t pays t o get t o know t hese standards, s ince the al ter nat ive is conformance test ing fai lure in t hetarget count ry. This requires the manufacturer t o redesign the product t o comply wit h the standards, w hich canbe an extremely cost l y task t hat dr amat ical ly increases t ime t o market .

When in Rome...

The best known safet y agency is Underw rit ers Laborat ories (UL). I t 's t he wor ld's largest safet y agency and has

had more components and f inished products submit t ed t o i t for test ing than al l other safety agencies combined.UL addresses every kind of elect rical and el ect ronic comp onent , eit her exclusively in standards addressing asingle product , or w it hin a larger standard t hat covers equipment design. Requirement s for power t ransform erscan be f ound in several UL st andards.

In Canada, t he Canadian Standar ds Associat ion (CSA) is responsibl e f or m aint aining a benchm ark f or safet y ofcomponents and products sold throughout the country. CSA has several standards for transformers, and theprocess of cert i f icat ion is di f f erent f rom t hat of UL.

In Europe, most countries abide by either a standard from the International Electrotechnical Commission (IEC)or have t heir own st andard b ased on IEC st andards.

Manufacturers must somet i mes also abide by t he rules of a state or l ocal agency that mandates requirements

specif ic to it needs. For example, the County of Los Angeles, because earthquakes are an inevitabil ity there,may require the use of current and thermal pr otect ion and non- f lammable mat erials, even though nat ionalstandards do not. Consequently, it 's always best to check all l ikely sources of requirements at the national andlocal level.

Getting to Know UL

When people see the UL tag on electrical and electronic equipment, they generally

say it's UL approved. However, UL doesn't use the word approve but rather has

separate guidelines for finished products and the components with which they'remade. If acceptable, complete products ready for the end user will be UL listed,

and components meeting UL specifications will be UL recognized. Since

transformers are fundamental electrical components, they can be UL recognizedif

they meet the requirements of an applicable UL standard.

This chassis-mount transformer meets

UL, VDE, CSA, and IEC standards and

provides isolation of 4 kV rms


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The fol lowing examples are r epresentati ve of UL standards:

UL506: This st andard covers only "specialt y" transform ers, which can be broadl y defi ned as small powert ransform ers. The st andard specif ies constr uct ion m ethods, dielect r ic st rengths, and a maximum operat ingt emperatur e r ise for power t r ansformer s rated 10 kVA or less. The great m ajor i ty of t ransform ers usedt hroughout electr i cal and electr onic products are covered by UL506.

UL1411: Transformers used in radio, television, and other similar appliances are covered by this standard,which is similar in its requirements to UL506. The primary differences are that UL1411 requires the use ofUL94V0-l isted, f l ame-retardant mater ial for term inal anchoring points and includes abnormal perform ancet est i ng according t o the t ype of f i nished product in w hich the t ransform er is used.

UL544: Medical and dental equipment is covered by this standard. As might be expected, UL544 is quitedetai l ed and str ingent in i t s requirement s, s ince the f inished product is of t en used in si t uat ions that are cr i t i calt o someone's health. In addit ion, t his equipment is of t en connected dir ect ly t o a pat ient , w hich makes isolat ionfrom hazardous voltages even more important. Also, UL544 includes a l imit on primary-to-secondary leakagecurrent and requires some means of int ernal t hermal or curr ent f using or a manufacturer-specif ied ext ernalcurrent fuse.

UL1585: This st andard covers Class 2 and 3 tr ansfor mer s, w hich are cur rent -l i mi t ed devices designed t oprotect the user from the consequences of an overload. The transformers covered by UL1585 can be either

inherent l y or non-inherent l y l imit ed. Inherent ly l im ited t ransform ers are designed so that , in the event of along-term overload, t heir im pedance wi l l be high enough t o keep them from being destr oyed and causing a f i re.Most wi l l cont inue t o operate properly w hen normal condit ions are restored. Transform ers contain a therm al orcurrent fuse or an external f use specif ied by t he manufacturer of the f inished product .

I t 's import ant t o note t hat a t ransform er manufactur er 's ent i re product l ine is not automat ical ly UL recognized i fone or mor e of it s indi vidual pr oduct s has met UL requi rem ent s. Cust om versions of exist i ng product s must besubmit ted to UL for r ecognit i on, w hich can take up t o 12 weeks depending on the governing st andard. Whenspecifying a certain product, ask the manufacturer if it 's UL recognized, and to which standard. Using UL-recognized t ransformer s is wort h any ef fort required t o obtain t hem because i t signif icant ly reduces the t imerequired t o get UL l ist i ng of t he end product .

More on CSA and IEC

CSA's st andard f or specialt y t ransformer s is C22.2, No. 66-1988. It covers design and im plem ent ati on of air-cooled, dry-type power transformers of 1,000 kVA or less operating with a primary voltage of 750 v or less. Thisst andard is very sim ilar t o UL506 and includes bell-r inging tr ansfor mer s and t ransformer s for Class 2 circuit s.There are differences also in the way testing is conducted, as well as in marking requirements. The Canadianstandard governing electromedical products (C22.2, No. 125) is quite thorough and similar to UL544.

In Europe, t here are many st andards, but fort unately t here is commonal i ty among them. The t wo mainst andards are IEC-950, m aint ained b y t he IEC, and EN60950, whi ch is the European Communit y version of IEC-950. An addit ional standard, IEC-601-1, is generally accept ed t hroughout Europe as t he st andard by whi chmedi cal elect ronic equi pment must comp ly (as UL544 is in t he United St ates and C22.2. No. 125 is in Canada).

The tr end in Europe is t o consolidat e st andards in order t o simp lif y the de sign and manufact ure and subsequentmarketing of products within the European Community. EN60950 is an example of this trend and is designed toprovide an umbrella standard that safety agencies in various countries can use either as is or with modif icationsthat satisfy local needs.

The German safet y agency, Verband Deut scher Electr ot echniker ( VDE) long ago const ruct ed a set of standardsthat has become widely respect ed thr oughout t he worl d. Many countr ies have adopted t he content of suchst andards as VDE0805 and VDE0750. However, as st andards consolidat ion cont inues, m ore count ries -- evenGermany -- are adopting IEC and EN standards.


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In short , ther e are quit e a few standards that specif i cal ly address the d esign and implement at ion of powert ransform ers. I t 's essent ial for the designer and the m anufacturer of elect ronic equipment to look at t he end useof t he equipment and accommodate t he requirement s of any appl icable standard at the beginning of the designcycle. The alternative can be a costly, embarrassing, t ime-consuming redesign.

Specifying the Proper Transformer

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