Zs on a RCD

[nicsin02]

Hi guys was just looking for a few answers,

1. If a Zs on a mcb fails but the whole D/B is protected by a 30mA RCD does this make the Zs results pointless, or does it still count as a C2 fault

2. I am working at a food distribution place and most of the outside sockets have failed the Zs test, all the sockets are used so that the lorries can plug in their refrigerator's over night, the mcb's are mostly BS60898 32a type D and most would pass going to a type C, I don't have my paperwork to hand just now so can some1 plz quickly go over the uses of type B,C,D with me and what would be most suitable for the lorries,



Thanks in advance

 

[kingeri]

1. The RCD in a TN system is for additional protection only - it should not be used as a substitute for installing circuits which will operate the breaker or fuse within the required time under fault conditions. On TT systems, things are a bit different, as it is usually impossible to meet the disconnection times, hence why we always need to RCD protect TT systems!

2. Is there a reason why type D's were used in the first place? Usually these would only be used where large inrush currents are expected - by fridges are you talking about a refrigerated lorry?

 

[kingeri]

By changing the breakers to Type C's you MAY find that this causes nuisance tripping. Nobody would use type D's unless absolutely necessary. Trouble is that the disconnection time is harder to meet with type D's. Best answer really would be to find a way to bring those Zs readings down!

 

[Amp David]

First off what the earthing system being used?

 

[nicsin02]

It is a TN-C-S system, point 1 and point 2 are not related to each other, there is no other way to get Zs down without rewriting cable,

 

[Amp David]

Never heard of supplementary then?

- - - Updated - - -

Must be some big compressor motors in them lorry fridges too

 

[nicsin02]

It's an outside D/b and its wired with SWA tie-wrapped to the fencing, SWA comes straight from the D/b to each point (radial) and the armour is bonded at both ends at D/b and onto the metal pole to which the socket is fixed, is their anyway of installing a spike to bring down the value of a circuit or does this have to be done at D/B, (Zdb wasn't high at all, only 0.07, cheers

 

[Amp David]

What size is the cable and how longs the run.

You will also need to find out the characteristics of the fridges on these lorries before you can think about swapping all D type out for C's

 

[cobrauk]

What are the readings you are getting at the sockets ? how long are the runs and size of cable etc

 

[malcolmsanford]

I really can't see where there is too much of a problem here.

Using an RCD for additional protection like is is completely within the regs (Section 415) and could very well have been designed like this to cut the cost of cable, if D type breakers were specified.

As David says there is also supplementary equipotential bonding available and even increasing the size of your cables, especially if SWA was you used you could have run a 3/5 core and combined the extra core with the armour to reduce your Zs values, but even these 2 methods may not have been enough or cost effective, and using an RCD was.

 

[loop3]

If the circuit is protected by a 30ma rcd the max zs is 1667 ohms. Have a close look at the regs.

 

[nicsin02]

The circuit isn't protected by a 30mA RCD, the RCD is at the end of the circuit protecting whatever gets plugged in, I thought an RCD is only for additional protection on a TN system, all supplementary bonding has been carried out, only options are re-wire cables or check and see if type C is suitable, all circuits protected by a 32a type d mcb

 

[Guest55]

max Zs for 32a D mcb is 0.36ohms and going to a C is 0.72
what was your Zs at the socket ?
if its still higher than this then the cables are way too small.
and the value of 1667 ohms only applies to circuits entirely rcd protected.

 

[nicsin02]

max Zs for 32a D mcb is 0.36ohms and going to a C is 0.72
what was your Zs at the socket ?
if its still higher than this then the cables are way too small.
and the value of 1667 ohms only applies to circuits entirely rcd protected.

Think at the top of my head the max Zs is 0.29ohms at 80% , the measured Zs range from between 0.34 up to 0.52, tested around 60 and 12 of them failed, next step maybe is to find out the load of the lorries and see if a 20a. Type D is suitable (if type D is needed)

 

[Engineer54]

I really can't see where there is too much of a problem here.

Using an RCD for additional protection like is is completely within the regs (Section 415) and could very well have been designed like this to cut the cost of cable, if D type breakers were specified.

I can, and yet another reason why i don't take the Reg's too literary!! To design a circuit that relies on the continuing serviceability of an RCD for it's earth fault protection is just asking for trouble down the road. Any circuit installation should be designed to fulfil the ADS of the protection device protecting that circuit. In this instance some of these circuits are clearly not achieving a suitable Zs to enable the protective device to operate in the specified times. It also seems that these circuits as a whole, are not RCD protected, only the loads (combined RCD/socket out-lets).

Far too much of this sort of thing going on these day's, where electricians are literary relying on RCD devices as the sole means of protection, to provide disconnection times where the ADS/Zs is woefully lacking. More often than not just by loose connections, or even connections that have all but come adrift, but can't be arsed to trace out and find them!!