Discuss 100mA delayed RCD tripping before the 30mA at DB. TT System in the Electrical Forum area at ElectriciansForums.net

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DaveyD

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Hello

Just not on the job at the moment but the builder called. He plugged his small angle grinder in to a new circuit and it took out the RCD at the supplying DB not the 30mA at local CU.

Here's the set up:


TT . 100A DNO fuse

House CU old and full so split tails into small CU with 100A/100mA time delay RCD feeding a CB . SWA to outbuilding/studio and duel RCD CU.

upstream RCD 100mA Timed x1 at 310ms

Both downstream RCD's 30mA checked as perusual and passed on all seven tests.

Electrode at house and new electrode at outbuilding/studio (New at 1.87 Ohms).

My question is ... why has the time delay rcd tripped and not the 30mA? Thank you in advance for your time.

Regards, Dave
 
Have you done IR tests?
Have you tried a ramp test of the time delayed RCD? Could be that there's some leakage current, but not enough to trip it until the grinder is plugged in. For example 80mA leakage + another 10mA from the grinder causing it to trip, but not taking out the 30mA ones.

And how did you get 1.87 Ohms on TT ? Is it a rod, or something else?
 
Hi. Thanks for your replies guys. It's a upfront 100mA timed and the studio cu is a duel board 2 x 30mA split load.
All insulation resistance tests plus 500Mohms.
Electrode is 2 x 3/8s coupled.
 
... Sorry ramp test. No not done as not on the job today and it all tested ok so I was not looking for faults. Will ramp when I get back.
 
The old house board has 100mA on all circuits and a further 30mA on selected. Typical old setup on tt.
This is not tripping.


I have split the tails into a new DB with 100mA delay rcd and a breaker to protect the submain to the studio.
 
Electrode is 2 x 3/8s coupled

Who put those in then?? That ground, must be more festering manure than soil!! Less than 2 ohms on a couple of short twigs, i doubt it!!

As for the S type 100mA RCD tripping before the 30mA standard RCD's, something is very wrong wouldn't you say? Time to get your test gear out, and find out why...
 
Who put those in then?? That ground, must be more festering manure than soil!! Less than 2 ohms on a couple of short twigs, i doubt it!!
Sorry my mistake 5/8 ths. I put them in. To be fair I only tested with a loop tester but ZS tests showed expected results too. What is the problem? Too LOW for you E45?

As for the S type 100mA RCD tripping before the 30mA standard RCD's, something is very wrong wouldn't you say? Time to get your test gear out, and find out why...
Thank you Sherlock :) The plumber agrees! I have tested but as explained I'm not on that job today and just wondered what others felt would make it trip upstream. Like a forum conversation. I will ramp when I get back there and replace if needed.
 
Last edited:
Sorry my mistake 5/8 ths. I put them in. To be fair I only tested with a loop tester but ZS tests showed expected results too. What is the problem? Too LOW for you E45?

With just a one metre depth, Yes you could say that!!
 
With just a one metre depth, Yes you could say that!!
They are 4ft each, I stated there were two coupled together - thats more than a meter <2.5m. What are you suggesting E54? I'm not offended by the way I just don't get where you are going with this? lol
 
They are 4ft each, I stated there were two coupled together - thats more than a meter <2.5m. What are you suggesting E54? I'm not offended by the way I just don't get where you are going with this? lol


Sorry my mistake, and pleased to hear that you have installed 2X Rods at both locations

You stated across 2 posts that you had 2 electrodes one for the house and another for the studio. and 2 X 3/8'' coupled. Well i know 3/8'' rods can't be coupled together so i've been taking it they were 2 X single rods...
 
Sorry my mistake, and pleased to hear that you have installed 2X Rods at both locations

You stated across 2 posts that you had 2 electrodes one for the house and another for the studio. and 2 X 3/8'' coupled. Well i know 3/8'' rods can't be coupled together so i've been taking it they were 2 X single rods...

OK, thanks. I was talking to an engineer (retired) tonight and he suggested that I might try isolating the earth electrodes from each other. I have them joined at both ends at the moment - found this IET Forums - Outbuilding Supply from House on TT System
 
We had this about a year ago on one of our clients farms , they also run a advents company there as well and erect marquess for it on the old tennis courts .
The supply has a MEM 100a , 0.1 amp s type at the head and a MEM 100a 00.3 at the marquee board . but the 0.1 s type would go first when a certain fridge was used .
After a bit of head scratching my other director who was dealing with this one decided to phone MEM technical , and i believe they said that if the the stake on the head had a significantly lower Ra than the one at the marquee then it would be enough to take out the s type first , he knocked a few more rods in to get it down and it has since been OK .
If i remember to ask him tomorrow for the exact readings involved i will post them up !
 
We had this about a year ago on one of our clients farms , they also run a advents company there as well and erect marquess for it on the old tennis courts .
The supply has a MEM 100a , 0.1 amp s type at the head and a MEM 100a 00.3 at the marquee board . but the 0.1 s type would go first when a certain fridge was used .
After a bit of head scratching my other director who was dealing with this one decided to phone MEM technical , and i believe they said that if the the stake on the head had a significantly lower Ra than the one at the marquee then it would be enough to take out the s type first , he knocked a few more rods in to get it down and it has since been OK .
If i remember to ask him tomorrow the exact readings involved i will post them up !

That's so helpfull of you DP - thank you I do appreciate it. That would be interesting. Regards, David
 
We had this about a year ago on one of our clients farms , they also run a advents company there as well and erect marquess for it on the old tennis courts .
The supply has a MEM 100a , 0.1 amp s type at the head and a MEM 100a 00.3 at the marquee board . but the 0.1 s type would go first when a certain fridge was used .
After a bit of head scratching my other director who was dealing with this one decided to phone MEM technical , and i believe they said that if the the stake on the head had a significantly lower Ra than the one at the marquee then it would be enough to take out the s type first , he knocked a few more rods in to get it down and it has since been OK .
If i remember to ask him tomorrow for the exact readings involved i will post them up !

They would need as you say, be significantly lower/different...
 

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