OSG question

[Albert]

In the OSG (blue) table 1A, page 85 is dealing with max demand and diversity table 1B page 86 deals with diversity. For lighting it does recommend different values, any explanation?

 

[steve190]

Discharge lighting uses a starter to ignite the gasses when switched on. This starter can draw up to 1.8 times the current need to sustain the ignited gas. Flourescent lighiting is classed as discharge lighting so for the calculations of total load then the possible starting current is the factor used when designing the circuit.


Thats what I was taught it to mean. Over to the experts

 

[Albert]

Many thanks
Albert

 

[Stoday]

Table 1A is about the current to allow for each outlet point, nothing to do with diversity between a number of points.

Table 1B is diversity between a number of points over a circuit.

 

[Albert]

So what it means is that using table 1A will always require higher values in point of view of current demand.

 

[kendor]

Discharge lighting uses a starter to ignite the gasses when switched on. This starter can draw up to 1.8 times the current need to sustain the ignited gas. Flourescent lighiting is classed as discharge lighting so for the calculations of total load then the possible starting current is the factor used when designing the circuit.


Thats what I was taught it to mean. Over to the experts

This is not what happens, the starter is merely a device to break the circuit for a short period of time and this break in the Heater circuit(the glow at the ends of the tubes you notice are the heaters when they are warming up the gases inside to aid starting)causes the choke(inductor) to induce a high voltage across the tube for a short period when struck a tube shows little or no resistance so a high starting current develops, this current flow through the choke which is in series with the tube causes a build up of what is called a back emf ( this happens in other inductive devices such as motors hence the high starting currents) when the back emf equalises the supply then the current is limited( normally called the operating current) so it is this initial starting current which needs to be calculated for.

 

[steve190]

And the value of this initial starting current is?

 

[kendor]

And the value of this initial starting current is?

dependant on the volts and the circuit impedance i would have thought.

 

[steve190]

Fair enough. I'll rephrase .

Why is there a correction factor of 1.8?