Hi Tim:
I'll assume the design meets the creepage and
clearance distances required by the standard.
Supposedly, the required clearances withstand
the expected overvoltages based on overvoltage
category. If so, then a transient overvoltage
should not cause an arc.
The required clearances are based on a purely
inhomogeneous field. In practice, the field
for most constructions is mid-way or better
between inhomogeneous and homogeneous fields.
So, the actual withstand voltage of a required
clearance is considerably more than the
required withstand voltage. So, even a
transient overvoltage exceeding the overvoltage
category should not cause an arc (breakdown of
the clearance).
According to IEC 60664-1 {ed1}, Annex A, Table
A1A, a 10 mm clearance should withstand a single
9 kV impulse (inhomogeneous field). A purely
homogeneous field will withstand a 25 kV impulse.
For typical construction, i.e., not purely
inhomogeneous and not purely homogeneous field,
I would expect a 10 mm clearance to withstand
15 kV peak or more.
I would not expect to see even a 9 kV impulse
on a 230 V mains.
I suggest you look for a root cause other than
an overvoltage. With 400 A, I would look for
a heat source such as contact resistance between
two conductors. I would hypothesize that the
Tufnol would be partially decomposed by the heat
source. Decomposition leads to highly reduced
insulation resistance, and then allows tracking
across the surface of the insulator. Then, an
arc can occur from the track to the steel, but
not enough current to immediately trip the MCB.
Best regards,
Rich
Richard Nute, CFEI
forensic investigations
11830 Wilmington Road
San Diego, California 92128
USA
Tel/FAX: +1-858-592-2620
Mobile: +1-858-776-1618
e-mail: [email protected]
[email protected]
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Behalf
> Of Haynes, Tim (SELEX) (UK Capability Green)
> Sent: Tuesday, February 27, 2007 7:56 AM
> To: emc-pstc
> Subject: Creepy Clearance distances....
>
>
> Hi All,
> I have a problem (not that one!)
>
> A piece of high current 230V 400A apparatus has failed with
> an obvious breakdown to ground that has resulted in an arc
> forming and burning through 2mm steel before the current
> caused the over current protection (400A MCB) to operate.
>
> Separation distance from conductor to earth is 10mm - part
> air, part Tufnell bush (spelling?).
>
> The Tufnell is burnt and the hole through the 2mm steel is
> right at the edge of the Tufnell bush.
>
> The only way I can see this failing, is to have had a large
> (lightning) transient on the supply causing a flashover that
> did not extinguish.
>
> (Comments on the above would be appreciated)
>
> However, the real question is - in the clearance distances
> (as from creepage and clearance) are they set so that when
> the over voltage (that causes breakdown) is removed the arc
> self extinguishes?
>
> Regards
> Tim
> 6239
> desk A1S77
> P Please consider the environment before printing this email.
>
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