Testing electrical installations

Details Testing electrical installations Testing electrical installations

1999-01-07

2001-08-07

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

For fault location

C324S710000

Reexamination Certificate

active

06271668

ABSTRACT:

BACKGROUND
This invention relates to the testing of electrical installations and in particular, to the testing of such installations which include residual current devices (RCDs).
To ensure the safety of an electrical installation a measurement is made of the impedance between the live or phase terminal and earth at various points: this test is commonly known as a ‘loop test’. Traditionally the loop test applies a load between live and earth such that a high current flows for a relatively short period of time (typically 25A according to International standards). The voltage drop due to the current is used to calculate the loop impedance.
Installations often include a residual current device (RCD) to detect a fault current flowing to earth by sensing an imbalance in the live and neutral currents flowing to the installation. An imbalance above a set level causes the RCD to ‘trip’, and disconnect the mains to the installation.
A loop test applied on the load side of an RCD applies a large current imbalance which trips the RCD. The loop test therefore cannot be completed and there may also be inconvenience to users as the mains supply fails.
EP-A-0295880(Robin Electronics) describes a method of testing which prevents an RCO from tripping while a standard loop rest is carried out. However newer RCD designs are susceptible to tripping even using this method.
Accordingly the present invention seeks to provide a method of performing a loop test without tripping RCDS.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a method of carrying out a loop test of a mains circuit which comprises applying a narrow pulse of a high peak value (for example 25A peak) in order to measure small loop impedances accurately by maximising the voltage drop during the pulse.
RCD designs include filter circuitry to prevent ‘nuisance tripping’ which would otherwise occur when a noise transient appears on the mains supply, for example when equipment is switched on. Accordingly, the width of the pulse is chosen to be not long enough for-an RCD to trip. Also, the shape of the pulse is such that the RCD circuitry is least likely to be sensitive to it.
Several pulses may be applied in order to complete the loop test measurement to the required accuracy. The interval between the pulses is designed to be long enough for the FCD to settle to its noqminal state, before the next pulse is applied.
Because a short pulse is used, the reactance in the system (typically the inductance of the substation transformer supplying the mains) can have a large effect on the test result.
According to a second aspect of the invention there is provided a method of carrying out a loop of a mains circuit test in which a signal of relatively low amplitude but containing high frequencies is injected into the circuit.
This signal contains frequencies significantly higher than mains frequency, for example audio frequencies of up to 10 kHz, which can thus easily be detected in the presence of mains. Also, higher frequencies produce larger phase shifts in the reactive component thus easing the measurement.
The signal is preferably applied for a number of successive cycles allowing transients in the system to decay then the phase angle of the signals voltage and current can be measured. Thus the method allows the resistive and reactive components of the loop impedance to be calculated.
Because timing is critical in this method, a digital circuit (which may include a microprocessor) is used to control the instrument. Depending on the test circumstances either or both of the above methods may be used. The instrument displays either the magnitude of the loop impedance, with or without the phase angle, or may display the resistive and reactive components separately or may display a fail/pass indication against a set level.
In addition to, or instead of, this the instrument may display prospective short circuit current, which is the current that would flow through a short circuit fault between live and earth at the time of the test.
It will therefore be understood that the invention also extends to a two-stage method of testing an installation in which one stage comprises applying a narrow pulse of a high peak value, and the other stage comprises injecting a high frequency, low amplitude signal, to enable the measurement of both resistive and reactive components of impedance.


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patent: 5969516 (1999-10-01), Wottrich

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