Service Fuse in Short Circuit?

As you all know, the REC does not care anymore as to what they connect to..


Has anyone experienced this type of incident? - I am not willing to try this one out

What happens when the REC man inserts a new 100amp service fuse straight onto a dead short circuit at the intake position? i.e. when a DIY'er assembles a busbar in his or her new consumer unit with both the Red and Black main switch terminals linked to the prongs in the forked bus-bar below the din-rail?

Also where the External impedance of 0.8.ohm comes from on a TNS system- 0.8. ohm seems low, when all that providing the earth is a bit of dodgy corroding lead covering on the cable?

i would expect a pretty big bang. possibly the fuse carrie prongs to weld to the service head aswell

A correctly erected TN-S system will have no dodgy clamps etc.

0.8 in my opinion is high - when do you ever record such a high reading!

0.8. is the standard accepted value as published in the Onsite Guide - but seems optimistic, seeing the condition of many incoming supply cables, with badly corroded coverings, i.e. when the pavememt is dug up by the REC to investigate a supply failure!

0.8 is the max allowed for earth loop impedance on a TN-S (in reality its often much lower)

especailly with lead and paper cables its not unheared of for the earth loop to be lower than the PN loop

iirc the main worry with using bonding clamps on lead sheathed cables is that the lead tends to migrate away leaving a dodgy connection which may well be severly dmaaged or destoryed in the event of a short.

as for plugging in a service fuse against a dead short i would have thought the rec would have made sure the CUs were switched off and the tail arrangement looked reasonable before plugging in the fuse.....

And of course if he is being inspected, he will be wearing a face shield, gloves and standing on a rubber mat. I dont think I've ever seen this away from the training ground. Has anyone else?

Quarter of an ohm is more like it on a lead sheathed TNS, unless the clamp has come off - the jacket cross-section is much larger then one phase core, at least when new.
Actually even 1/4 ohm is only 1000A, which will do a fair bit of damage, being comparable to a lorry starter motor current, but most breakers are rated to open safely at least once at 6KA these days.
The real damage is usually done on trying to break the connection, as an arc of hot metal and ionised gas is formed at the point of interruption. By comparison, making the contact is quite mild.
I believe the rubber mat and all that is really in case the head comes off the wall and causes a short before the company fuse, I guess in a really grotty instalation that might be possible.
regards M.

Kai - have you got nothing better to do than to think up increasingly bizarre hypothetical situations and pointless questions about the size of old ceiling roses?

Please go back to Screwfix and talk about pocket seams.