So I'm In A House Today And I Notice A 30ma Rcd On The Wall

[OnlQQker]

It feeds an outdoor socket. On the comsumer unit is a 60ma Rcd and everything is covered by this.

I'm just wondering if the 30ma Rcd would trip before the 60ma Rcd trips (if that makes sense)

It's just my mind wondering about stuff, the chap who owns the house said it's never tripped out, yet I wondered if 30 ma would trip first because it's lower than the 60ma in the consumer unit. To me I'm guessing it would?
Anybody helping explain this would mean I might get some sleep tonight without thinking about it in bed, I've tried searching the net for the answer but no luck!

 

[telectrix]

60mA RCD? are you sure?

 

[Andy78]

60mA RCD ? Would be the first I've heard of.

 

[Pete999]

60mA RCD? are you sure?

R U sure it's 60ma?
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60mA RCD ? Would be the first I've heard of.

Likewise

 

[Julie.]

Depends totally on the makes of the rcds

Once the current gets around 5-10x the trip setting, they don't get any quicker.

So if the 30mA one trips in 10.4mS (5x), but the 60mA goes in 10.2mS (5x), then the 60 would trip first due to the fault current normally being well above both mcb's 5x values; as the fault current is usually many amps (to meet normal disconnection times) so both are in a race to trip, and once they have started they will finish...

So whichever one trips first is purely academic as they will both operate in similar times

 

[OnlQQker]

60mA RCD? are you sure?

Not now, it was only a glance because high up. Will make an excuse to have a look again tomorrow. Now if it is a 30ma they will both trip right, or would the closest one go first?

Old Hager consumer unit (approx. 10 years old) , kitchen ring, upstairs ring, down ring, garage, cooker. Lighting is on an RCBO
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Depends totally on the makes of the rcds

Once the current gets around 5-10x the trip setting, they don't get any quicker.

So if the 30mA one trips in 10.4mS (5x), but the 60mA goes in 10.2mS (5x), then the 60 would trip first due to the fault current normally being well above both mcb's 5x values; as the fault current is usually many amps (to meet normal disconnection times) so both are in a race to trip, and once they have started they will finish...

So whichever one trips first is purely academic as they will both operate in similar times

Many thanks for explaining that Julie. It was one of them curiosity things more than anything!

 

[Andy78]

If they are both 30mA you could ---- a coin as to which will go first.
Have a play with a RCD tester and see what happens.

 

[Julie.]

Phoenix used to supply 60mA rcds, but just din rail, and not the normal thing to put in a hager cu.

There is absolutely no coordination or discrimination between rcd unless there is a time delay, both will trip in around the same time,

 

[littlespark]

Ramp test any 30mA rcd and you’ll get different measurements. There’s no reasoning if one is closer to the load or not as to which would trip first.
And if one is “aged” or otherwise of a lesser quality, the tripping threshold could change day by day. As long as it trips before 30mA is reached

 

[Vortigern]

I have two RCD in line one at the main DB and two RCBO Lights and socket, in the garage. The garage is 20m from the house on TN-C it trips in the garage if tested. How about that! And lets not get into whys or wherefores on the matter it is just a fact and that is how it is.

 

[telectrix]

I have two RCD in line one at the main DB and two RCBO Lights and socket, in the garage. The garage is 20m from the house on TN-C it trips in the garage if tested. How about that! And lets not get into whys or wherefores on the matter it is just a fact and that is how it is.

aha. trips when tested. may not do that in a real fault scenario.

 

[OnlQQker]

I have two RCD in line one at the main DB and two RCBO Lights and socket, in the garage. The garage is 20m from the house on TN-C it trips in the garage if tested. How about that! And lets not get into whys or wherefores on the matter it is just a fact and that is how it is.

Thanks for this reply.
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aha. trips when tested. may not do that in a real fault scenario.

I thought testing was to stimulate real fault scenarios?

 

[telectrix]

it is to simulate, but simulation does not always work in real life. viz. Titanic.

 

[OnlQQker]

it is to simulate, but simulation does not always work in real life. viz. Titanic.

Sorry I meant 'simulate' not 'stimulate' thanks for the explanation though!

 

[Moley]

it is to simulate, but simulation does not always work in real life. viz. Titanic.

I thought it was an iceberg that sunk the Titanic. I didn't realise it was an rcd

 

[davesparks]

Not now, it was only a glance because high up. Will make an excuse to have a look again tomorrow. Now if it is a 30ma they will both trip right, or would the closest one go first?

An RCD mainswitch will probably go first as it will likely have some normal earth leakage from all of the connected circuits biasing it to trip first.

 

[ipf]

I thought it was an iceberg that sunk the Titanic. I didn't realise it was an rcd

Always thought it was bloody big floating lettuce...!

 

[littlespark]

An RCD mainswitch will probably go first as it will likely have some normal earth leakage from all of the connected circuits biasing it to trip first.

Not always. As I mentioned before, an rcd could trip at anything up to 30mA.
Circuit could trip at 19, main switch at 29... fault current may sit at 24..

It’s possibly down to whichever rcd is the most awkward to reset.
e.g a static caravan has an rcd mainswitch in its consumer unit, there is also a 16A rcbo in the hookup box outside.
if there’s a fault, it’s 9/10 that it’s outside that trips..... in the middle of the night...... when it’s raining.....

 

[R-fur]

The only way to make RCDs discriminate is to put a time delay on the upstream one,