April 2013 Newsletter Page 2

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Key Risk Items When Upgrading Control Systems

Read June 2013 Edition


Specific Advice for Those Doing Upgrade to Legacy Systems with Metasys by Johnson Controls

Read June 2013 Edition


CAS BACnet Object Monitor – SubscribeCOV

Read June 2013 Edition


USA Tax Deduction and Credits Expires Jan 2014

Read June 2013 Edition


Accuracy and Power Metering

The transducer used to measure current and energy and power is a current/instrument transformer (CT). Thus the accuracy of these measurements is dependent on the accuracy of the CT. It is also dependent on the accuracy of the electronics measuring the secondary current but since most electronics are highly accurate and stable this consideration is minor.

Accuracy is comprised by the actual input to output transfer ratio, as well as linearity and phase shift. Accuracy is comprised by the actual input to output transfer ratio, as well as linearity and phase shift.

The accuracy of a CT is dependent on:

  1. Design Factors – Burden, Rating, Type
  2. Installation considerations – Load, Metering Cables
  3. Environmental conditions – Noise, Temperature , Humidity, Age, Fault Currents

Figure 1 – Moulded Case

Moduled_Case

Figure 2 – Split Core CT
Split_Core_CT

Burden

A CT has a burden resistor applied. The secondary current flowing through this resistor provides the measurable voltage for metering purposes. The burden needs to be selected to ensure the voltage produced does not saturate the core of the CT. How you hook up the secondary to the meter, i.e. the wire, length etc will add to the burden. In calculating the load presented by the cable, ensure you double the length since the current must run their and back. The combined burden for cabling and meter must not exceed the VA rating of the CT.

Figure 3 – Meter Cable Length / Burden
Meter_Cable_Length_Burden

Linearity and Knee Point

The relationship between the primary and secondary current should be linear (straight line proportional) in the chosen operating range for accuracy purposes. The point at which non-linearity is reached, when currents are high,  is known as the Knee-Point. Typically this applies when CT’s are used in protection (as opposed to metering) devices since the anticipated peak current might be 20x normal.

Linearity and Light Load

Light load is 10% of nominal current. Special CT’s are typically required when accurate measurements are required with light loads.

Temperature

Affects the linearity. Take care to choose a CT rated for the operating environment. Areas with excess heat, cold or lack of air flow may need special considerations.

Electromagnetic Noise

Type

Solid and Split Cores (don’t need a shutdown to install but expensive) and non-core CT’s. Split core’s are less accurate because of the gap in the winding.

Polarity

Only important for measuring power. CT must be installed in the correct orientation. Current flowing into the CT does so at the point marked with a DOT or label ‘H1’. The dot should face the ‘source’ side. Secondary terminals are typicall marked X1 and X2.

Fault Currents

Are often much higher than normal operating currents and the CT should be selected with this in mind both from an accuracy point of view and also to ensure the installation is not dangerous.


Native Mounting of ISO and VHD Files in Windows 8 Explorer

Windows 8 Explorer now supports native mounting of ISO and VHD files. Accessing ISO and VHD files has become just two clicks away, without having to install any third party software to open or mount them.

By double clicking the ISO file, Windows 8 Explorer automatically creates a removable drive which allows you to access the content straightaway. When you want to unmount the file, just click Eject in the Windows 8 Explorer’s ribbon menu and the virtual drive will disappear.

VHDs appear as new virtual hard drives in My Computer when mounted.

This significant change in Windows 8 also allows Windows 8 tablet users to copy ISOs to their device via USB and gain access to the information very easily.

You can backup your entire DVD collection to ISOs and store the files on your local hard disk for easy access.

The only downside is that the Windows Explorer only reads the ISO and VHD files, it cannot create them.

Watch Accessing Data in ISO and VHD Files by Rajeev Nagar.


Testimonial from Tieder Control

Recently our engineer, Alex Fontaine, had supported our client Tieder Control and received a positive feedback.

“I appreciate your time and dedication. I just wish other manufactures/vendors had such good quality support like you have given.”

– Andy Stinson

Thank you for your positive feedback, we will continue to provide excellent support to our clients!


Can 25kV Electrostatic Discharge Kill You?

The evidence in this video suggests you will survive but not all your neurons will.

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