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  • Users: charz
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  1. charz

    Arc Resistant Switchgear

    When to buy an arc resistant switchgear? If I buy an arc resistance switchgear, Can I open the controls compartment of HV or LV switchgear without PPE? Am I safe when I open the door/panel of one de-energized compartment? While the adjacent compartment remains energized. Do I still need to wear...
  2. charz

    Gas group IIA vs IIB, IIC

    How common (as a standard product) is the availability of gas group Ex de IIA, Ex ec IIA motors? As I could see from our past projects, most LV motors are with gas group IIB, so IIB looks more common. The concern here is that vendor is offering gas group IIC for some motors; if I accept the IIC...
  3. charz

    SCCR rating

    My specification calls for a short circuit rating of 35kA rms and 77kA peak value for a junction box with a bus bar in it. The vendor proposes a junction box with a short circuit tested at 90kA for 0.1s. He justifies that peak mechanical effects (potential damage) to a product due to a short...
  4. charz

    Hazardous Area Certification - Saudi Aramco

    The Saudi Aramco specification says, 8.3.1 IEC or Ex labeled equipment/component meeting requirement of IEC 60079 and certified by one of the agencies in the approved IECEx certification bodies (ExCBs) under IECEx Certified Equipment Scheme is acceptable. Method of protection must be marked...
  5. charz

    Exciting Current of CTs - Differential / REF

    The attached article says that the relay setting should be above the sum of the exciting currents in the parallel CTs. ( I got this article from one of the old threads on this same forum). Does it mean the exciting currents of the CTs circulates through the relay path? how does the exciting...
  6. charz

    CT Polarity - In differential protection

    Is the attached arrangement of CTs correct for differential protection of transformer? The "P2" of CTs on both the primary & secondary of transformer are towards the transformer. https://files.engineering.com/getfile.aspx?folder=f1a521f2-e0c4-4b5f-8b42-8004cea93d6e&file=rkXxQ.png
  7. charz

    High Impedance REF - Stabilizing resistor

    The stabilizing resistor in high Impedance REF assists in such a way that the derived current in the relay circuit (Rst + Rp) cannot reach the relay setting threshold when the maximum through current saturates a CT with its DC component. Generally, the faults closer to neutral have less...
  8. charz

    Grounded Wye - Grounded Wye VT Question

    What are the implications of primary neutral of a VT not connected to the ground in both the ungrounded and grounded system? The configuration of VTs are three separate single phase VTs connected grounded wye - grounded wye fashion?
  9. charz

    Crack on Surge Arresters

    Almost all the surge arresters in all the feeders of MV switchgear (6kV) got cracks as shown in the picture. What are the reasons for it apart from dirt, moisture and inadequate capacity?. The said surge arresters were in operation for just more than a year. We have planned to replace it with...
  10. charz

    Broken Delta Secondary of VT

    The broken delta connection of the secondary of the VT should be grounded or not while connecting to the damping resistor. Some manufacturer catalog shows grounding connection and some not.
  11. charz

    Local/Remote Switches in MCC

    Is it normal to have Local / Remote switches in both MCC and in LCS near the Motor? I think it is possible to have misoperation with Local / Remote switches in both location. Is it necessary/code requirement to have Local/remote switches, start/stop push buttons in the MCC? as I would like to...
  12. charz

    Vacuum Contactor - Specifications

    Following is the technical specifications of vacuum contactor in which I have some questions. 1.Does it mean that the protective relays can be used to trip the vacuum contactor till 12,500A?, then why Maximum break capability is mentioned as 8000A? 2.What is the difference between the...
  13. charz

    Vibrations in buildings/Structures - Effects in Electrical systems.

    I have earlier posted about the failure of capacitors in a pumping installation vide thread Link. Recently we have a observed a pattern of loosening of bolts/nuts in several starter panels where the capacitors are mounted. Under these structures discharge lines passes through for a short line...
  14. charz

    Vacuum Contactor Coil

    The attached picture is control circuit of a solenoid latching vacuum contactor. It has an electronic unit to control the coils. 1) What is the purpose of this electronic unit? 2) Why the 52b contact is connected to the coils as I understand it is solenoid latching coil. Once the power goes...
  15. charz

    HVAC Motors - Name Plate Details

    Why the HVAC vendors do not provide name plate details of the Motor, Instead they provide only MCA (Minimum Circuit Amperes), MOCP (Maximum Over current protection), RLA ( Running Load Amperes), LRA (Locked Rotor Amperes). Suppose if the ACCU has " 2 compressors + 6 condenser fan's + some...
  16. charz

    Cable Route guidelines

    How to approach cable routing for HVAC works? Should it be started after or before the duct layout has been prepared? How the clashing between cable tray and duct, pipelines can be avoided?
  17. charz

    Applications of Latched type VCs

    Where are the latched type VC's are used? Are latched type Vacuum Contactors with fuses are equivalent to VCB's? In reactor starter motor feeder, one manufacturer has used latched type VC's? Is there any disadvantage of latched type as opposed to the electrically holding type Vacuum contactor in...
  18. charz

    Failing Capacitors

    Problem: We have a motor driving a vertical pump. The problem is there is no check valve at the discharge, so whenever the pump is stopped, due to the back flow the shaft rotated for about 40 seconds. The excitation to the motor is provided the power factor capacitor. The Power factor of the...
  19. charz

    VFD Problem

    The Power to the 1050kW, 6kV motor is fed from VCB-->VFD-->Motor. We heard from the operator that the motor stops suddenly and starts after the 30s~1min. I feel that this might be due to the voltage fluctuations from the power supply side. The Under voltage protection at the VCB Panel has 50ms...
  20. charz

    Electronic Trips in MCCB

    There are LSIG, LSI, LS, LI type electronic trip units in MCCB. I understand that it provides Long-time, Short-time, Instantaneous, Ground fault type of protection depending on the type of electronic trip unit. One would normally prefer LSIG type electronics trip unit as it offers all the...

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