Forums
New posts
Search forums
What's new
New posts
New media
New media comments
Latest activity
Media
New media
New comments
Search media
Members
Current visitors
Log in
Register
What's new
Search
Search
Search titles only
By:
New posts
Search forums
Menu
Log in
Register
Install the app
Install
Main Forums
Student & Learning Zone - City & Guilds
What fault can turn a TT into a TNCS and disable the RCD?
JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser
.
Reply to thread
Help Support Talk Electrician Forum:
This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Message
<blockquote data-quote="PC Electrics" data-source="post: 263401" data-attributes="member: 650"><p>For those having difficulty following the "whys and wherefors", let me try and simplify it for you:</p><p></p><p>Lets say we have a neutral path back to the star point of 0.2ohm.</p><p></p><p>And lets say that the earth path back to the star point (via our rod) is 100ohm</p><p></p><p>And lets say our N-E fault is a dead short, so 0ohms.</p><p></p><p>We have a total load of 10A on our circuit.</p><p></p><p>Now, the ratio of resistances between the neutral and earth paths is 1:500 (0.2:100)</p><p></p><p>So, on its way back to the star point our 10A current reaches the N-E fault point. Here it can 'see' two paths back to the transformer. The current will split in the same ratio as the resistances, ie 1:500. Thus 0.02A will flow down the earth path, whilst the remaining 19.98A will flow down the neutral path.</p><p></p><p>As can be seen, the fault current of 0.02A (20mA) won't be sufficiently high to trip an RCD.</p><p></p><p>If however the system were a TN, we would have essentially the same resistance back to the transformer down both neutral and earth paths. thus our 10A would have split roughly in half. With a 5A imbalance an RCD would trip.</p><p></p><p>Does that help?</p></blockquote><p></p>
[QUOTE="PC Electrics, post: 263401, member: 650"] For those having difficulty following the "whys and wherefors", let me try and simplify it for you: Lets say we have a neutral path back to the star point of 0.2ohm. And lets say that the earth path back to the star point (via our rod) is 100ohm And lets say our N-E fault is a dead short, so 0ohms. We have a total load of 10A on our circuit. Now, the ratio of resistances between the neutral and earth paths is 1:500 (0.2:100) So, on its way back to the star point our 10A current reaches the N-E fault point. Here it can 'see' two paths back to the transformer. The current will split in the same ratio as the resistances, ie 1:500. Thus 0.02A will flow down the earth path, whilst the remaining 19.98A will flow down the neutral path. As can be seen, the fault current of 0.02A (20mA) won't be sufficiently high to trip an RCD. If however the system were a TN, we would have essentially the same resistance back to the transformer down both neutral and earth paths. thus our 10A would have split roughly in half. With a 5A imbalance an RCD would trip. Does that help? [/QUOTE]
Insert quotes…
Verification
Post reply
Main Forums
Student & Learning Zone - City & Guilds
What fault can turn a TT into a TNCS and disable the RCD?
Top