Time/Current characteristics for....

[Mikey_A]

6A Type B circuit breaker to BS EN 60898.

The 'current for time' in BS7671 is 0.1 to 5 seconds disconnection time for 30A of current. I'm looking at the chart (fig 3.4 on page 199) and the 'time,seconds' line actually goes upto approx 12 secs. I don't understand why?

 

[Spark123]

The curve above the line you speak of is the thermal part of the MCB, once the current gets up to 30A the magnetic part of the MCB should operate and cause it to trip within 0.1s.

 

[Jim2287]

instantaneous only at 5x current and above... less than 5x current takes time to disconnect (thermal part as spark says)


If your earth loop isnt low enough to allow 30 amp (5x current) through then it doesnt comply with regs... therefore thermal part for overload protection rather than fault current



5 x 6amp = 30 amps required for 6 amp 60898 to operate in time

240 x 30 = 8 ohms max earth loop for 30 amps to flow

48 / 6 = 8ohms, max earth loop (without correction page 46 brown regs)

 

[Driller]

It's hard to see but there is a vertical part of the line that runs down from your "12 seconds" to 0.1 seconds. It's only slightly thicker than the grid lines.

If you look at the other values its a bit more obvious.