ANALYSIS OF THE QUASI PEAK DATA

CHAPTERS

5.2 ANALYSIS OF THE QUASI PEAK DATA

the rain, supporting the concept of a saturation point at which very little additional ionisation by electron impact takes place on a particular conductor bundle in air. The fire-induced corona noise activity showed stepped increases in the noise level from the lowest surface gradient recorded at 9 kV/cm. The level of noise generated at 9 kV/cm due to the gas fire corresponded to the level of rain-induced corona noise generated at 17 to 18 kV/cm and was 27dB/lV higher than the rain- induced corona noise at 9 kV/cm. The noise generated above 11 kV/cm is higher than the rain- induced corona noise at any gradient during the test. The audio noise at the time of these measurements was high and the ensuing flashover is an indication that no further increases in ionisation without a full breakdown across the existing 3.5m gap were possible. Instead, the corona region about the conductor had extended down the heat column (or "plasma") and developed from corona activity into leaders which then bridged the gap between the conductors and the earth plane. This was remarkably repeatable at 17 kV/cm, stressing the fact that when the surface gradient was high enough and the environmental conditions were appropriate for sustained heat conditions at the conductors, the transition from ionisation to corona to leaders was consistent and inevitable. However, as the environmental conditions are rarely stable, such

"appropriate" conditions were unlikely to persist for all the work performed in the corona cage.

A significant result here (not directly related to the detection of fires) is the repeatable insulation breakdown across the air gap with no particles and no significant contribution of any additional chemical reactions from any flammable materials other than the flammable liquid petroleum gas (LPG).

5.2.3 Twin Zebra

The RN noise recorded for the rain and normal dryconditions were as anticipated according to predictions and other measurements.

The fire-induced noise recorded on the analogue quasi-peak instrumentation was erratic. The measurements do not correlate with previous knowledge of this work and the optical alignment of the flame was identified as the fundamental cause for the irregular measurements. The operator was requested to ensure that the flames were directly below the conductors before recording the noise levels. The operators control room was situated at ninety (90) degrees with the burner and overhead conductor bundle. Therefore the flames would rise towards the conductors directly in front of the operator. However, due to the fact that the flame was

operator to detennine the precise position of the flame. The result was that the flames were, on occasion, vertically behind the conductors though appearing to be directly below the conductors.

As previously discussed, in such measurements, the level of noise should be similar to that recorded in the nonnally dry conditions. The "bad" data could easily be identified as that data in close proximity to the dry condition data. In order to gain an improved approximation of the fire- induced corona noise generated, a "predicted" curve from the original data was extrapolated. The predicted trend is illustrated in figure 5.6 below.

The fire-induced corona noise generated from the twin Zebra conductor bundle was recorded at increasing intervals and was not as vigorous as the fire-induced corona activity occurring on the twin Dinosaur conductor bundle. The fire-induced corona noise levels were approximately 5 to 13 dBIlV higher than the rain-induced corona noise levels when using the predicted values. With or without the projected figures, the twin Zebra data was not as convincing as the twin Dinosaur data although the fact that the fire-induced corona noise was more active than the rain-induced noise was significant. Mfects such as the length of conductor exposed to the specific condition may significantly change these results. This comparison of conductor types is discussed in a later section.

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5.2.4 Triple Wolf

At 12 kVfcm, the fire-induced corona noise activity was more vigorous than the highest level of rain-induced corona activity measured, which was at 26 kVfcm. The same trend towards what appears to be a saturation point was observed as the rain-induced corona noise had smaller increments with each kVlcm increase in the surface gradient. The rate at which the dry condition noise was increasing (relative to the surface gradient) indicated that the two curves were likely to intersect at some higher gradient. The fue-induced quasi-peak level corona activity was much higher. The results drew attention to the significant difference in quasi-peak levels. In the range in which a comparison was possible, the rain- and fire-induced RIV levels were separated by between 12dBIlV and 25dB1lV.

5.2.5 Twin Wolf

At 10 kVfcm the fue-induced corona noise activity was equivalent to the rain-induced corona noise activity generated at 16 kVfcm and 14dBIlV greater than the rain-induced corona noise activity generated at the same surface gradient of 10 kVfcm. The trend was virtually the same for all three conditions. A maximum separation of 21dBIlV between the rain- and fire-induced corona noise was recorded at 12 kVfcm. Again, further increases in the surface gradient for further tests may have resulted in a flashover taking place.

5.2.6 Single Wolf

As for the twin Zebra conductor bundle, the entire range of fire recordings made in the single Wolf tests, appeared to be "subdued", with typical dry condition levels recorded at 24 kVfcm.

The fire-induced corona noise data was consistently higher than the rain-induced corona noise data (altering the 24kVfcm result as for the Twin Zebra test as illustrated in the graph below).

The largest difference in quasi-peak noiseIevels was 1 7dBIlV at 12 and 16 kVfcm. Over the entire range of surface gradients the results appear to have been very "smooth" even when taking the adjustment at 24 kVfcm into account.

On occasion the gas pressure from the gas burner dropped or the gas flow was reduced due to some freezing on the pipes taking place. This usually resulted in the reduction of the flame plasma height and a reduction in the thermal influences experienced at the conductors in the

the overall temperatures will be lower every time the gas burner is operated under these conditions. The results obtained on other conductors certainly did not support the "smooth"

increase in fue-induced corona noise experienced here.

Single Wolf Conductor Bundle

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Figure 5. 8: Modified Single Wolf quasi-peak data in accordance with theory.