Siemens AC-3 Datasheet download pdf (Page 33)

Bimetal Overload Relays 3UA and 3UC

The 3UA / 3UC thermal overload relays are suitable for customers from all industries, who want

guaranteed optimum inverse time delayed protection of their electrical loads. The relays meet the

requirements of IS/IEC 60947-4-1.

Application

3UA overload relay: 3UA5/6 are 3 pole adjustable bi-metal

overload relays mainly suitable for normal starting applications.

They provide accurate and reliable protection to motors against

overload as per CLASS 10A. They also offer protection against

single phasing and unbalanced voltages.

3UC overload relay: 3UC5/6 are 3 pole adjustable, saturable CT

operated bi-metal overload relays mainly suitable for heavy

starting applications (i.e. when heavy masses are to be put in

motion resulting in larger starting period). They provide accurate

and reliable protection to motors against overload as per CLASS

30. They also offer protection against single phasing and

unbalanced voltages.

If single-phase AC or DC loads are to be protected by the 3UA /

3UC thermal overload relays, all three bimetal strips must be

heated. For this purpose, all main current paths of the relay must

be connected in series.

Standards

Bimetal relays conform to IS/IEC 60947-4-1. They also carry the CE

mark.

Range

3UA5: 0.1 to 120A, (Class 10A, without CT)

3UA6: 85 to 630A, (Class 10A, CT operated)

3UC5/6: 2.4 to 400A (Class 30, CT operated)

Relay overview

Overload relay operates on the bi-metallic principle. The heater

winding wound on the bimetal strips carry the current flowing

through the motor. In case of overload, the current carried

through the heater winding is more than the rated current. This

heats up the bimetals. Due to this bi-metal strips bend and open

the NC contact of the relay, which is connected in the control

circuit of the contactor, thus disconnects the motor from the

supply. The tripping time is inversely proportional to the current

flowing through the bi-metal strips. Bi-relays are therefore,

referred to as “current dependent” and inverse-time delayed

relays.

1. Connection for mounting onto contactors:

Optimally adapted in electrical, mechanical and design terms

to the contactors, these connecting pins can be used for direct

mounting of the overload relays. Stand-alone installation is

possible as an alternative (in some cases in conjunction with a

stand-alone installation module).

2. Selector switch for manual/automatic RESET (blue):

With this switch you can choose between manual and

automatic RESET. A device set to manual RESET can be reset

locally by pressing the RESET button. A remote RESET is

possible using the RESET modules (accessories), which are

independent of size.

3. TEST button (red):

Trip circuit can be manually checked by using this button.

During this simulation the NC contact (95-96) is opened and

the NO contact (97-98) is closed. This tests whether the

auxiliary circuit has been correctly connected to the overload

relay. The relay must be reset with the RESET button if it has

been set to manual RESET. If the thermal overload relay has

been set to automatic RESET, then the overload relay is

automatically reset when the TEST button is released.

4. Motor current setting dial:

Setting the device to the rated motor current is easy with the

large rotary knob. (Recessed dial, hence no possibility of

accidently change in current setting.)

5. Trip indicator (Green):

A separate mechanical Green Trip Indicator is provided on the

front cover of the relay to indicate the tripped state of the

‘manual reset’ relay.

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Siemens AC-3 Datasheet Page 33

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