Frequently asked question on Siemens breakes

Q1) Must the neutral conductor always be connected to the left pole of the ACB?
ANS: On 4-pole circuit-breakers, the neutral conductor must always be connected all on the left.
Otherwise this can cause malfunction of electronic over current release.

Also for non-automatic breakers the neutral always has to be connected to the left phase (due to the design layout of the breaker).


Q2) Are the circuit breakers 3WL able to detect and evaluate:
• Power flow direction?
• Energy flow direction?
• Reverse current flow?
ANS: For the circuit breaker 3WL with electronic tripping unit ETU45B or ETU76B, equipped with the metering function (and voltage transformer) there are extended protective functions available as described in the operating instructions (revision 11.2006) on page 9-100.
These functions can be used for tripping the breaker..
These signals can be displayed on the ETU or made available as floating contact using a digital output module.
Using COM15, these data are accessible via Profibus-DP.

Please be aware that add-up values are resettled when the ETU is disconnected from the auxiliary supply.
Either, these functions - in addition to the delay time as given in the operating instructions - require a response time of about 1 second.


Q3) Is ground-fault detection with 3WL ACB in an IT-System feasible?
ANS: No, it is not possible to achieve ground-fault detection with 3WL ACB in an IT-System.
One has to use an insulation monitoring device for ungrounded networks.
These devices permanently monitor the insulation resistance between the ungrounded single or three-phase AC-supply and a protective conductor and instantaneously signal occurring faults.

We recommend the use of a 3UG insulation monitoring relay for ungrounded networks




Q4) Which maintenance and inspection work can be carried out on a circuit breaker?
ANS) Information about maintenance of the circuit breaker is included in the operating instructions, part 24; also it is given in separate link on this page. Furthermore maintenance and inspection have to be done according the requirements of the switchboard manufacturer or the end user.

Apart from an inspection of the contact system and the arc cutes the circuit breaker basically is maintenance-free.
Within the routine maintenance of the breaker some operations should be done (if there were no switching operations during service) to move the mechanical system.

Independent from this an inspection has to be done after any short-circuit tripping.



Q5) How is ground-fault protection achieved with 3WL circuit-breakers?
ANS: Vectorial summation current formation (method 1 - ETU27B or higher):
The three phase currents and the N-conductor current are measured directly.
The electronic trip unit determines the ground-fault current by means of Vectorial summation current formation for the three phase currents and the N-conductor current.
For 4-pole breakers all CTs are integrated, for 3-pole breakers an additional external CT for the neutral protection is required.
This Rogowski-coil 3WL9111-0AA2.-0AA0 or 3WL9111-0AA3.-0AA0, named -T5 in our documents, has to be wired to terminals X8.9 and 10 as shown in the operating instructions on page 9-124 (edition 11.2006).

Direct measurement of the ground-fault current (method 2 - ETU45B or higher):
A standard transformer with the following data is used for measurement of the ground-fault current:
1200 A/1 A, Class 1, 15 VA (5 A secondary cannot be used).
The transformer can be installed directly in the grounded neutral point of a transformer.
This CT (T6 in our documents) has to be wired to terminals X8.11 and 12; the polarity of the CT does not effect the detection.

Q6) What is the selection criteria for 3WL circuit breaker used for protection of capacitors?
ANS)According to IEC, capacitor units have to be appropriate for operation with a current, whose r.m.s value does not exceed 1.3 x the current flowing at sinusoidal rated voltage and rated frequency.
Furthermore, there is an excess of 15 % over the real value of the energy tolerated.
The maximal current for switching and permanent load for the circuit-breaker shall be 1.5 x nominal rated capacitor current.


Q7)For which applications of ACBs 3WL and MCCBs 3VL are the very low settings of the short time delayed short circuit protection Isd = 1.5 x Ir useful?
ANS)With the aid of the short time delayed short circuit protection Isd it is possible to inhibit trappings as consequences of rejection inrush currents or current spikes.
The specific setting value for Isd depends mainly on the existing application.
There exist generators with very low short circuit currents.
That’s why the short time delayed short circuit protection is provided with setting options down to Isd = 1.5 x Ir.
Furthermore the different Isd-setting options are useful for application specific discrimination considerations.



Q8) How can the ETU (electronic tripping unit) be activated to make the parameterization?
ANSWER:
The correct state of the over current release is displayed by the green flashing "Active" LED in heart-beat rhythm.
The "Active" LED starts flashing at:

- current of 80A for frame size I and II,
- current of 150A for frame size III or
- supply of 24V DC auxiliary voltage (from ETU 45B)

When the current exceeds set operating current of over current release (I >= Ir), then yellow "Alarm" LED on the ETU turns on.

Notice:
The "floating state" between activated and non-activated over current release, at load-currents close to activation-limit of circuit-breaker with ETU45B is displayed with the message "Waiting for Trip Unit".

Due to this, the protection function is ensured even without additional supply voltage. The power supply is provided by the internal current transformer.

Tripping characteristic shows the behavior of the over current release when it is activated via a current before the release. If release (instantaneous short-circuit release) happens immediately after switching on the breaker and due to this, the over current release has not activated yet, the opening time takes about 3 to 10ms longer.



Q9) May the 3WL circuit breaker be used in systems of e. g. 150 HZ or 400 HZ?

ANSWER:
The electronic trip unit is designed for 50/60 HZ +-10 % - there were no tests for other frequencies.
At higher frequencies the energy transformers would overload the power supply unit of the ETU.

For 400 HZ the 3WN6 e. g. with trip unit type "B" (but not "N" or "P") can be used.
At this frequency the breaker has to be oversized per 30-40% due to higher eddy-current losses.
May be that for 800 A a breaker 1250 A has to be used, but the setting of the trip unit may remain at 800 A.


Q10) Can a 3WL be used with a voltage above 1000V?
ANSWER:
Breakers 3WL12/3WL13 with option A05 (Ue = 1000 V) were tested with a voltage of 1000V +5% =>1050V.
They were not tested at any higher rated voltages Ue.
Therefore it is not allowed to use 3WL breakers for Ue > 1050 V maximum value.


Q11) Are there negative effects to be expected in the operation of 3WL with frequency converters?
ANSWER:
Frequency converters generate harmonics and often cable-born noise voltages, mostly common-mode interferences, in the line supply.
Reasons are:
- Low-frequency harmonics caused by 6-pulse supplies.

- Switching reactors on the line side, causing high voltage peaks.
- Switching the IGBT of the converter with very fast switching times.
- Resonant circuits producing high voltage peaks.

Most important is to have EMC-correct design of system configurations:

- Metal cabinet parts must be connected together and must be grounded.
- Signal cables and power cables must be routed separately.
- Always use shielded signal cables.

Low-frequency harmonics are reduced using line reactors.
The electronic system of the ETU calculates the rms-value and is therefore suitable for systems being afflicted with harmonics. The ETU is tested according EN 60947-2, Annex F, including electromagnetic compatibility (EMC) tests, immunity tests and harmonics.

Attention should be paid to the use of frequency converters in IT-systems. In non-grounded line supplies these peaks cannot discharge to earth via PE/PEN conductor.

For ACBs switching frequency converters, especially in IT-systems, we suppose the use of an additional EMC-filter for the ETU. This filter is connected between CT and ETU and reduces noise voltages.

Ordering: Order code Z=F31 or separately by using order-no. 3WL9111-0AK32-0AA0.


Q13) How about the function of the thermal memory?
ANSWER:
Thermal memory of 3WL operates in following way:
Shortly before tripping energy value data responding to the actual current flow are read into a clock unit and the time counter of the clock module is resetted.
The buffered clock unit automatically continues operating.
When re-energizing the energy value data as well as the time counter are read out from clock device and converted in values which are interpretable for protection functions.


Q14) Does the display of the ETU45B of the circuit breaker 3WL an auxiliary supply to show the measured currents or is the minimum current of 80 A resp. 120 A sufficient ?
ANSWER:
If ETU45B is not supplied with auxiliary power while minimum current is present, then background light is deactivated.
The display, however, is active and shows the measured currents.