Indirect Contact rotection Using RCDs?

[Danny Huht]

Could anyone please explain why an RCD is needed outside the equipotential zone, how it operates and also why it needs to operate with in a certain specified time ?

 

[Pete999]

Could anyone please explain why an RCD is needed outside the equipotential zone, how it operates and also why it needs to operate with in a certain specified time ?

 

[hightower]

Is this a college question? What are your thoughts on these questions?

 

[gazdkw82]

Isn't direct and in-direct contact a redundant term now? Replaced with basic and fault protection?

 

[Danny Huht]

Is this a college question? What are your thoughts on these questions?

Well I know RCD's work on the principle of measuring the balance in current across the live and neutral which indicates a leak to earth when unbalanced. Operation time obviously need to be very low in order to prevent electrocution, but as for the first bit I'm stumped.

 

[Danny Huht]

Wont let me watch it at the moment as I'm at work but i'll be sure to watch it later, thanks
Its more so the first question I'm stuck with, any info?

 

[Danny Huht]

Isn't direct and in-direct contact a redundant term now? Replaced with basic and fault protection?

Quite possibly, it's just how its worded on an assignment

 

[hightower]

Well the point of an equipotential zone is to ensure that any touch voltage is within a safe limit (normally no greater than 50v). However, if you step out of the equipotential zone you don't have that safety and are subject to higher voltages. So to take a lawn mower as an example: you grab a faulty cable, and since the weather was really nice you decided to do it bare foot. Take a resistance through you of 1000ohms as an example, the current through you and to the ground would be (in simple theory) around 0.23A - more than enough to kill you, yet not enough to operate the MCB protecting the circuit. You'd be toast.

The same theory could apply to an unbonded water pipe within the house. A standard (L-E or L-N) fault will operate the breaker, but because in the above example you make up part of the circuit, you have a resistance that restricts the amount of current that can flow. The breaker doesn't see an issue, even though there is. An RCD will detect this and break the circuit, hopefully, before you become toast.

 

[Danny Huht]

Well the point of an equipotential zone is to ensure that any touch voltage is within a safe limit (normally no greater than 50v). However, if you step out of the equipotential zone you don't have that safety and are subject to higher voltages. So to take a lawn mower as an example: you grab a faulty cable, and since the weather was really nice you decided to do it bare foot. Take a resistance through you of 1000ohms as an example, the current through you and to the ground would be (in simple theory) around 0.23A - more than enough to kill you, yet not enough to operate the MCB protecting the circuit. You'd be toast.

The same theory could apply to an unbonded water pipe within the house. A standard (L-E or L-N) fault will operate the breaker, but because in the above example you make up part of the circuit, you have a resistance that restricts the amount of current that can flow. The breaker doesn't see an issue, even though there is. An RCD will detect this and break the circuit, hopefully, before you become toast.

Spot on explanation, couldn't of been clearer. Cheers mate

 

[FatAlan]

Personally I wouldn't operate a lawn mower in bare feet. All sorts of stones and other debris tend to shoot out. ;-) Succint as ever Hightower. I reckon you're destined for a job in teaching!

 

[hightower]

Personally I wouldn't operate a lawn mower in bare feet. All sorts of stones and other debris tend to shoot out. ;-) Succint as ever Hightower. I reckon you're destined for a job in teaching!

Wouldn't have the patience mate. And if you've never mowed the lawn bare foot, you've never lived

 

[FatAlan]

You haven't seen my lawn!