Electrocution TN-C System

[maypot]

Hi,

A casual worker was superficially burned on the fingers when a power cable was accidently put into contact with the frame. The worker was touching the frame at this particularly instant. The 63 Amps MCB did trip but was not fast enough to prevent the accident.
According to the IEC rules, RCDs are not to be used on TN-C systems.RCDs should ONLY be used in the TN-S part of the system, meaning after splitting the PEN-condutctor into one PE- and one N-conductor. As a summary, RCDs should not be used in systems with PE- and N-conductors combined (as for the TN-C).
According to me, the RCD can be omitted in TN-systems as long as you can prove that the MCB will disconnect within a certain amount of time According to IEC60364, for TN-systems and protection against electrical shock:
413.1.3.3: It is said that Zs*Ia <= U0, i.e. system-impedance multiplied by the minimum current that assures that the breaker is disconnected within a given time should be less than the nominal phase-voltage.
The time is given as a table, and for U0=230V (i.e. TN Ull=400V) the maximum time is 0,4 seconds.
If the minimum fault current that occurs is quite large, and the system-impedance is kept quite low, you should be able to fulfill this requirement with a MCB, depending on it's caracteristics.

Bob

 

[ScottyUK]

Bob,

You're absolutely correct about RCDs being of no use in a TNC system - but where would you find one? The only TNC networks I can think of in the UK are the utility PEN supply, and they become TNC-S from the utility metering point so the customer side is a discrete neutral and earth. I've never heard of an RCD being applied in the TNC side of a supply, and I think the reason is that such an installation would be pointless - is this an actual installation you are describing, or hypothesis?

Is the second part of your post a question or a statement? It appears to be correct either way.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

Excuse me but is not electrocution by definition DEATH caused by electricity? How can you have a superficial death?

I presume you meant shock?

 

[maypot]

itsmoked,

I should have used the FRENCH word `ELECTRISATION` instead of electrocution which means death but I could not find the translation in english of electrisation.
TN-C networks are very common in my country and my post was related to the necessity of RCD in the installation.

Bob