PARTIALLY TYPE TESTED ASSEMBLY (PTTA)

IEC 60439-1, defines a PTTA as:

‘A low-voltage switchgear and controlgear assembly, containing both type-tested and nontype-

tested arrangements provided the latter are derived (e.g. by calculation) from type tested

arrangements which have complied with the relevant tests’.

 

This means:

• The assembly will be derived from a Type Tested Assembly.

• Deviations from the tested configuration are only permitted provided they can

be verified by calculation or equivalent methods. Such methods have their

limitations which must be fully understood and, where appropriate, allowed for

through safety factors in the analysis.

• As a means of demonstrating design compliance with the Standard,

manufacturers must have available Type Test documentation for the original

design and any calculations relating to modified elements.

• The Assembly will consist, as far as practical, of a series of standard elements

configured to suit a particular application. Customisation should be restricted to

situations where it cannot be avoided, e.g. facilities for terminating over size

cables, use of an alternative tested component, etc.

• All components and devices will be fully Type Tested to their respective product

standards. Documentation will be available for examination.

• Combinations of Type Tested components and devices installed in an otherwise

untested Assembly are not covered by this definition and the Standard.

 

(BEAMA Installation)

TYPE TESTED ASSEMBLY (TTA)

IEC 60439-1, defines a TTA as:

 

‘A low-voltage switchgear and controlgear assembly conforming to an established type or

system without deviations likely to significantly influence the performance, from the typical

ASSEMBLY verified to be in accordance with this standard.’

 

This means:

• The generic design of the Assembly including all the various elements used in its

construction have completed the Type Tests detailed in the Standard.

• Deviations from the tested configuration are permitted, but they will be very

minor and not ‘likely to significantly influence the performance’ e.g. a functional unit

may have been short circuit tested in the top of the Assembly and in service be

located lower down.

• Design verification must be via stringent testing, and does not rely on subjective

assessments, calculations, safety margins and engineering judgements.

• The Assembly will consist of a series of standardized elements including

interconnections, all of which have previously been proven by tests in the Type

Tested Assembly(s), but may be configured differently to suit the application.

• Assemblies may be built by licensed assemblers under strict controls.

 

(BEAMA Installation)

Current ratings

Rated current (In)

 

value of current, declared by the ASSEMBLY manufacturer taking into consideration the ratings

of the components, their disposition and application, which can be carried without the

temperature-rise of various parts of the ASSEMBLY exceeding specified limits under specified

conditions

 

Rated peak withstand current (Ipk)

value of peak short-circuit current, declared by the ASSEMBLY manufacturer, that can be

withstood under specified conditions

 

Rated short-time withstand current (Icw)

r.m.s value of short-time current, declared by the ASSEMBLY manufacturer, that can be carried

without damage under specified conditions, defined in terms of a current and time

 

Rated conditional short-circuit current (Icc)

value of prospective short-circuit current, declared by the ASSEMBLY manufacturer, that can be

withstood for the total operating time (clearing time) of the short-circuit protective device

(SCPD) under specified conditions

Voltage ratings

Rated voltage (Un) (of the ASSEMBLY)

The ASSEMBLY manufacturer shall state the rated voltage(s) necessary for correct functioning

of the ASSEMBLY.

 

Rated operational voltage (Ue) (of a circuit of an ASSEMBLY)

 

If different from the rated voltage of the ASSEMBLY, the ASSEMBLY manufacturer shall state the

appropriate rated operational voltage of the circuit.

The maximum rated operational voltage of any circuit of the ASSEMBLY shall not exceed its

rated insulation voltage.

 

Rated insulation voltage (Ui) (of a circuit of an ASSEMBLY)

 

The rated insulation voltage of a circuit of an ASSEMBLY is the voltage value to which dielectric

test voltages and creepage distances are referred.

The rated insulation voltage of a circuit shall be equal or higher than the values stated for Un

and for Ue for the same circuit.

NOTE For single-phase circuits derived from IT systems (see IEC 60364-5-52), the rated insulation voltage

should be at least equal to the voltage between phases of the supply.

 

Rated impulse withstand voltage (Uimp) (of the ASSEMBLY)

 

The rated impulse withstand voltage shall be equal to or higher than the values stated for the

transient overvoltage’s occurring in the system(s) to which the circuit is designed to be

connected.

IEC 61439 series

 

Electrical checks

Functional tests consist in checking the correct functioning

of all the circuits (electrical and electromechanical)

by simulating, as far as possible, the different service

conditions of the assembly.

For example, tests on current and voltage circuits can

be carried out by supplying the secondary circuits of

the CTs and VTs, without disconnecting the CTs from

the circuit.

Electrical checks may include the verification of the proper

operation of circuits and equipment, in particular:

- control, signaling, alarm, trip and reclosing circuits;

- lighting and heating circuits, if present;

- protection and measuring circuits (overcurrent, overvoltage,

earth, residual current trip units, contactors,

ammeters, voltmeters, etc.);

- terminals and contacts available in the terminal box;

- insulation control devices (also creepage distances

and clearances must be verified at level of connections

and adaptations carried out at workshop).

To carry out these checks, in addition to the normal mechanical

tools used for assembling, also some electrical

tools are necessary. A periodical calibration is necessary

to obtain reliable results.

The tools generally used are:

- a tester or multimeter;

- a test bench (AC and DC) to supply the assembly during

the test of the operation under voltage;

- a torque wrench (to check that the correct tightening

torques have been applied to the connections) and

other tools.

Mechanical checks

They shall be carried out complying with the relevant documents,

making reference to the following specifications:

- correct assembling of the equipment (connections

and, on a random basis, proper tightening of the connections);

- positioning and tightening of nuts and bolts;

- mechanical locks and controls (rack-in locking devices,

mechanical interlocks, key interlocks and

manual operating mechanisms for the removal of

circuit-breakers and switch-disconnectors by using

the operating levers and accessories provided with

the assembly);

- closing and possible blocks of the doors and adhesion

of the dust-proof seals to the assembly structure.

Further checks during testing

Visual inspections

They are carried out visually taking into account:

a) compliance of the assembly with diagrams, designations,

drawings and type of enclosures, number and

characteristics of equipment, cross-sectional area of

conductors and presence of identification marks on

cables and devices (initialing, inscriptions on plates,

etc.);

b) presence of components which allow the degree of

protection (roofs, seals) and the absence of faults on

the enclosure (cuts, perforations which might jeopardize

the degree of protection) to be guaranteed;

c) compliance with the specific prescriptions, if required

in the assembling list, such as:

- coating or treatment of busbars (resin coating, silver

plating, etc.);

- type of cable (fireproof, ecological, etc.);

- completion spare parts;

- painting check (color, thickness, etc.).

Routine verifications in compliance with

Routine verifications represent the final technical intervention

of the assembly manufacturer before the delivery

of the switchgear assembly completed and before

invoicing and shipment to the customer. The Standard

describes the verifications in the following order:

- Degree of protection IP provided by an assembly

enclosure

It represents the first routine test prescribed by the

Std. IEC 61439-1.

Actually it is reduced to a visual inspection.

- Clearances and creepage distances

Clearances usually results, also at visual inspection,

quite higher than necessary.

As regards creepage distances, reference shall be

made to the values defined by the Standard (shown

in Table 9.6, clause 9.2 of this Technical Application

Paper); for further details reference shall be made to

clause 12.6 of the Technical Application Paper, “Routine

verification of impulse withstand voltage”.

- Protection against electric shock and integrity of

the protection circuits

It is based on a visual inspection and on some verifications

of the correct mechanical tightness on a random

basis. The proper realization of the protective circuit

is verified:

- visually (e.g. checking of the presence of devices

which guarantee contact for earthing conductor

continuity etc.);

- mechanically (checking of connection tightness on

a random basis);

- electrically (verification of the circuit continuity).

The tools used are a tester and a torque wrench.

- Incorporation of built-in components

The real correspondence of the installed equipment

with the assembly manufacturing instructions is

checked.

- Internal electrical circuits and connections

Verification on random basis of correct tightening of

terminals is required.

- Terminals for external conductors

Correspondence of cables and terminals is checked

according to the wiring diagram.

- Mechanical operation

On a random basis levers, pushbuttons and any possible

mechanical actuating element are operated.

- Dielectric properties

See clause 12.6.

- Wiring, operational performance and function

The nameplate is checked and, if necessary, electrical

operation and any possible safety interlocks shall be

verified by test.

Protection against direct contact

Protection against direct contact can be obtained both

by means of the assembly construction itself as well as

by means of complementary measures to be used during

installation.

The protective measures against direct contact are:

- Protection by insulation of live parts

Live parts shall be completely covered with an insulation

which can only be removed by destruction.

This insulation shall be made of suitable materials

capable of durably withstanding the mechanical,

electrical and thermal stresses to which the insulation

may be subjected in service.

Paints, varnishes, lacquers and similar products used

alone are generally not considered suitable for providing

adequate insulation for protection against direct

contact.

- Protection by barriers or enclosures

All external surfaces shall provide a degree of protection

against direct contact of at least IPXXB.

Horizontal top surfaces of accessible enclosures having

a height equal to or lower than 1.6 m shall provide

a degree of protection of at least IPXXD.

The distance between the mechanical means provided

for protection and the live parts they protect shall not

be less than the values specified for the clearances

and creepage distances.

All barriers and enclosures shall be firmly secured

in place. Taking into account their nature, size and

arrangement, they shall have sufficient stability and

durability to resist the strains and stresses likely to occur

in normal service without reducing clearances.

- Protection by obstacles

This measure applies to open-type assembly.

Classification of assemblies

Assemblies may be classified according to different

Factors :

by the constructional typology, by the external design, by

the installation conditions, by the function carried out.

 

Open-type and enclosed assemblies

 

According to the constructional typology the Standard

IEC 61439-1 distinguishes between open-type and enclosed assemblies.

-  Enclosed assembly

An assembly is enclosed when there are protected

panels on all its sides so as to provide a degree of

protection against direct contact not lower than IPXXB

(see Chapter 4). Assemblies intended to be installed

in common environments shall be of enclosed type

 

-  Open-type assembly

 

An assembly, with or without front covering, in which

the live parts of the electrical equipment are accessible. Such assemblies can be used only in places

where skilled persons have access for their use.

UL Standards (Electrical enclosures)

Industrial Control Panel Enclosures

Products Listed under this category include enclosures that are intended to house open type industrial control panels.
The basic standards used to investigate this category is UL 508A, the Standard of Safety for Industrial Control Panels, UL 508, the Standard of Safety for Industrial Control Equipment, and UL 50, the Standard of Safety for Enclosures for Electrical Equipment.

Industrial control panels

Products Listed under this category include industrial control panels, which are factory wired assemblies of industrial control equipment such as motor controllers, switches, relays and auxiliary devices. The panels may include disconnect means and motor branch circuit protective devices. This category also covers industrial panel enclosures that are intended to house open type industrial control panels or individual items of industrial control equipment as noted above.

The basic requirements used to investigate this category are contained in UL 508A, the Standard of Safety for Industrial Control Panels.