# Why spark will occur when you switch on any device?

Discussion in 'General Electronics Chat' started by smslca, Jun 24, 2010.

1. ### smslca Thread Starter Member

May 11, 2009
26
0
When you switch on any electrical appliance you will observe some spark in the switch. Do you why spark will occur?
Air’s dielectric strength is 20Kv/cm. That means when you apply 20Kv voltage between two terminals and the distance between those terminals is 1 cm then dielectric strength of air will be broken between these two terminals and air will behave like conductor. Then you can observe the spark there. In normal house applications the voltage commonly used is 230V. So if the gap between terminals is 0.01cm then in that gap air will conduct for 230V voltage. At that time spark will occur.

2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,395
1,196
that is correct..

Jul 7, 2009
1,585
142
Your logic sounds right. Two minor nits: 1) the dielectric constant of air is usually taken to be around 3 kV/mm and 2) you might want to get in the habit of using correct SI units: instead of Kv, use kV. Minor, I know, but the units are either correct or incorrect. A common mistake is that many engineers write milliseconds as mS, yet that is a completely different unit: millisiemens. The careful reader then has to do a double-take and ask if the writer was talking about time or conductance...

Wife: get your narrow-little anal retentive butt on outta here -- nobody likes a nit-picker...
Me: Yes, dear, leaving now...

4. ### Ghar Active Member

Mar 8, 2010
655
73
The voltage isn't really 230 V. First of all that's RMS, meaning the peak (the one that matters) is 1.414 * 230 = 325 V

The other complication is that of inductance. As you plug things in the contacts touch and separate several times. This brief contact is enough to initiate current and when there is enough inductance in the path the current can't drop to zero fast enough when the contacts disconnect. The inductance will raise the voltage across the air gap will rise until the air breaks down.

5. ### Darren Holdstock Active Member

Feb 10, 2009
262
11
You'll get a bigger spark when you switch stuff off. This is back-EMF from the inductance in the electrical wiring, which can produce thousands of volts from some non-ideal arrangements. It's a big problem when switching inductive loads, such as electrical motors, and the switch contacts have to be massively derated accordingly.

When I was a kid, I was with a friend who was messing around with a car battery and a car horn. He got a big shock upon disconnection, and was perplexed as to how this could happen from a 12 V source. It was many years before I found out the answer to that particular puzzle.