GB2097942A

GB2097942A - Short circuit current protection

Info

Publication number: GB2097942A
Application number: GB8212988A
Authority: GB
Original assignee: Dorman Smith Switchgear Ltd
Current assignee: Dorman Smith Switchgear Ltd
Priority date: 1981-05-06
Filing date: 1982-05-05
Publication date: 1982-11-10
Also published as: GB2097942B

Abstract

A simple and inexpensive apparatus for determining the prospective short circuit current at any position in an electrical installation comprises means for measuring and storing the peak voltage of a half cycle of the operating voltage between conductors at said position, switching means for connection and disconnection of an impedance between said conductors to cause a current to flow therebetween for a short period; means for synchronising and sequencing operation of the peak voltage measuring means and actuation of the switching means; means for measuring the difference in voltage caused by the current flow; an indicating device and, associated with the indicating device, means for processing the voltage difference and applying it to the indicating device for a period of sufficient length to enable the voltage difference to be displayed. Preferably the indicating device has a scale so calibrated that a direct reading of the prospective short circuit current can be given. <IMAGE>

Description

SPECIFICATION Short circuit current protection This invention relates to the protection against short circuit current between conductors of an electrical installation.

National and International Standards, in particular the 1 5th Edition of the IEE Wiring Regulations require that protective devices be provided in an electrical installation to break any short circuit current in conductors of each circuit of the installation before such current can cause danger due to thermal and mechanical effects produced in conductors and connections. To meet this requirement it is increasingly being specified that the prospective short circuit current at every relevant position of the complete electrical installation shall be ascertained.

It is an object of the present invention to provide a simple and inexpensive apparatus for determining the prospective short circuit current at any position in an electrical installation.

According to the invention the apparatus comprises means for measuring and storing the voltage at a predetermined point (e.g. the peak voltage) of a half cycle of the operating voltage between conductors at any position in an electrical installation; switching means for connection and disconnection of an impedance between said conductors to cause a current to flow therebetween for a short period; means for synchronising and sequencing operation of said voltage measuring means and actuation of said switching means; means for measuring the difference in voltage caused by said current flow; an indicating and/or recording device and, associated with said device, means for processing said voltage difference and applying it to said device for a period of sufficient length to enable the voltage difference to be displayed and/or recorded.

Preferably, the indicating device has a scale so calibrated that a direct reading of the prospective short circuit current can be given.

In employing the apparatus of the invention to determine the prospective short circuit current at any position in an electrical installation, the value at a predetermined point (e.g. the peak value) of a half cycle of the operating voltage between conductors at the position in the electrical installation is measured and this voltage measurement is stored. A current is then immediately caused to flow between the conductors for a short period, which current is of sufficient magnitude to effect a difference in the value of the operating voltage at the position and, which voltage difference is a function of the prospective short circuit current at the position, and the voltage difference is displayed and/or recorded to provide an indication of the prospective short circuit current at the selected position in the electrical installation.

Over the range of short circuit currents and power factors encountered in practical electrical installations, the value of prospective short circuit current will be sufficiently accurate to enable an appropriate protection device to be provided for any relevant point in an electrical installation without reducing the level of safety or incurring any unnecessary expense.

The means for processing the voltage drop and applying it to the indicating and/or recording device preferably includes an amplifier.

The apparatus may be powered from the mains or batteries. Preferably, the switching means includes a thyristorto switch a current of the order of 1 00A through the impedance, which is preferably an accurate resistance.

Preferably, the apparatus has the facility to operate both on line and phase voltages, and for this purpose, a voltage level detector and indicators are provided.

The apparatus preferably also includes at least one protective device e.g. a fuse.

The apparatus is preferably mounted in a portable case to which can be connected leads for connecting the apparatus between two conductors of an electrical installation. Preferably, the case is divided internally into two separate compartments, one of which contains all the components of the apparatus operating at the installation voltage and the other of which contains the remainder of the apparatus.

Preferably, also, separate access is provided to each compartment of the case of maintenance and servicing.

The invention is further illustrated by a description, by way of example, of preferred apparatus for determining the prospective short circuit current at any position in an electrical installation with reference to the accompanying drawings, in which: Figure 1 is a vector diagram illustrating the mathematical principle on which the apparatus operates, and Figure 2 is an annotated block diagram of the preferred apparatus.

Referring to the vector diagram shown in Figure 1, where: V5 is the source voltage of an electrical installation V, is the voltage of the apparatus Vsz is the voltage across the source impedance VSR is the voltage across the source resistance Vsx is the voltage across the source reactance PF is the power factor of the source, and 1sc is the prospective short circuit current at any position of an electrical installation, from the vector diagram, it will be seen that and

If the resistance of the apparatus is R, it can be shown that::

Thus, if V, is measured and the values of R1, PF and Vs are either known or assumed, a direct relationship exists between V1 and 1sc and the scale of the indicating device of the apparatus measuring Vl can be calibrated in I.

The construction of the preferred apparatus will be apparent from the annotated block diagram shown in Figure 2 and from the following description of the operation of the apparatus.

With the apparatus connected to a voltage supply and two protruding leads of the apparatus connected to two conductors at a position in an electrical installation at which the prospective short circuit current is to be measured, the two voltage indicators will identify the line or phase condition and thus the nominal voltage to be related. When the apparatus is switched on, the measuring and logic circuits are energised and, when the "calibrate" button is actuated, the apparatus operates as a suppressed zero voltmeter. Any difference between the actual supply voltage and its nominal value is corrected by adjusting the variable gain amplifier so that the needle of the indicating device aligns with a "calibrate" mark at its full scale deflection.After calibrating the apparatus, the measure push button is actuated to initiate, via the control logic, the following sequence of operations:- 1. The peak detector and sample and hold circuits are re-set; 2. A signal is sent to prepare the trigger circuit to fire the thyristor switch; 3. When the trigger circuit is prepared, the control logic initiates firing of the trigger circuit and sampling of the peak voltage during the half cycle for which current flows through the impedance of the apparatus and the thyristor switch.

4. Where necessary, the logic operations are synchronised with respect to the supply voltage wave form by means of the zero crossing detector and pulse shaper circuits.

5. Firing of the trigger circuit also initiates a timer which inhibits any further triggering during this "time out" period, and also provides a 'hold' indication. Inhibition of any further triggering prevents excessive thermal loading on the impedance of the apparatus thereby enabling it to cool between each measuring operation of the apparatus.

6. After the perspective short circuit current has been displayed and/or recorded, the "calibrate" push button is again actuated. If the needle of the indicating device still aligns with the "calibrate" mark on the scale then the reading of this prospective short circuit current is valid since variations in the supply voltage are unlikely to have taken place.

Claims

1. Apparatus for determining the prospective short circuit current at any position in an electrical installation, which apparatus comprises means;for measuring and storing the voltage at a predetermined point of a half cycle of the operating voltage between conductors at any position in an electrical installation; switching means for connection and disconnection of an impedence between said conductors to cause a current to flow therebetween for a short period; means for synchronising and sequencing operation of voltage measuring means and actuation of said switching means; means for measuring the difference in voltage caused by said current flow; an indicating and/or recording device and, associated with said device, means for processing said voltage difference and applying it to said device for a period of sufficient length to enable the voltage difference to be displayed and/or recorded.

2. Apparatus as claimed in Claim 1, wherein the indicating device has a scale so calibrated that a direct reading of the prospective short circuit current can be given.

3. Apparatus as claimed in Claim 1 or 2, wherein the means for processing the voltage drop and applying it to the indicating and/or recording device includes an amplifier.

4. Apparatus as claimed in Claim 4, wherein the switching means includes a thyristor to switch a current of the order of 1 OOA through the impedence.

5. Apparatus as claimed in Claim 4 or 5, which includes at least one protective fuse.

6. Apparatus as claimed in any one of the preceding Claims, which includes the facility to work on both line and phase voltages and has a voltage level detector and line and phase voltage indicators.

7. Apparatus as claimed in any one of the, preceding Claims, which apparatus is mounted in a portable case to which can be connected leads for connecting the appartus between two conductors of an electrical installation.

8. Apparatus as claimed in Claim 7, wherein the case is divided internally into two separate compartments, one of which contains all the components of the apparatus operating at the installation voltage and the other of which contains the remainder of the apparatus.

9. Apparatus as claimed in Claim 8, wherein separate access is provided to each compartment of the case.

10. Apparatus for determing the prospective short circuit current at any position in an electrical installation substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.