RCCBO's and computers

[tangerinepiemuncher]

I am having a problem with some display units for computers fed via RCCBO's. Each display unit has a single supply running to it fed via a 20A 30mA RCCBO. The display units have anywhere between 12-20 computers/laptops on them and are switched on via a single switch. The problem is when the units are switched on the RCCBO's trip. I have measured the start up currents on the circuits using my clamp meter and no circuit is overloading. I know that upon start up computers can generate a high leakage current so I suspect this is why the RCCBO's are tripping upon switch on. How do I get round this problem?

 

[ban-all-sheds]

Don't switch them all on at once, or have fewer on the same circuit.

Do you know why the RCBO is tripping?

Is your clamp meter a sample-and-hold one which is quick enough to capture start-up surges?

Are you a qualified electrician?

 

[electronicsuk]

Twenty computers on a 30mA RCD/RCBO is simply asking for trouble. One way or another, you need to decrease the number of machines on each circuit, or find a way around the requirement for RCD protection.

 

[tangerinepiemuncher]

Cheers guys. Thought the answer would be reduce the number of PC's on the circuits.

 

[jimsky007]

you could also look into doing away with the RCCBO's BS7671 makes allowances for circuits used by trained staff no requirement for RCD protection also the way the supply is run to the PC's could be altered ie Surface trunking with socket outlets built in could be run around the office

 

[PrenticeBoyofDerry]

you could also look into doing away with the RCCBO's BS7671 makes allowances for circuits used by trained staff no requirement for RCD protection also the way the supply is run to the PC's could be altered ie Surface trunking with socket outlets built in could be run around the office

Surely it applies to skilled supervision of the electrical installation rather than trained staff.

 

[bernardgreen]

Surely it applies to skilled supervision of the electrical installation rather than trained staff.

Where trained staff are using electrical equipment in "controlled" environments it is not essential to fit earth leakage protection on the final rings.

At least one government laboratory has it's benches and several other work places supplied by final rings without RCD protection. This allows equipment to have fully filtered power supplies without creating RCD trips.

Equipment which may present a shock hazard has local RCD protection.

 

[ericmark]

411.3.3 Additional protection
In a.c. systems, additional protection by means of an RCD in accordance with Regulation 415.1 shall be provided for:

(i) socket-outlets with a rated current not exceeding 20 A that are for use by ordinary persons and are intended
for general use.

An exception to (i) is permitted for:
(a) socket-outlets for use under the supervision of skilled or instructed persons, e.g. in some commercial or
industrial locations.

However you have a duty to protect and I would be wary of removing any RCD device already fitted. Using UPS can help and that's how I go around the problem. Although there are some earthing issues with UPS which as yet I have not resolved.

I am uncertain about earthing requirements when power has been removed.

I have seen isolation transformers used and again I have questioned this as to my mind the supply after the transformer is a TN-S supply and it would require RCD's after the transformer.

But this is not a perfect world and one has at end of the day make a risk assessment and decide which root to take.

 

[bernardgreen]

I have seen isolation transformers used and again I have questioned this as to my mind the supply after the transformer is a TN-S supply and it would require RCD's after the transformer.

I have seen proven and recommended the use of a 230 to 230 isolation transformer with the secondary ( output ) centre tapped and this centre tap forming the isolated supply Neutral from which the isolated supply's "earth" is obtained. Whether this is connected to true earth ( as for computor equipment noise reduction or the earth of the incoming supply will depend on local requirements. This provides isolated 230 volts as two 110 volt phases 180 degrees apart. An RCD taking the two phases through it sensor provides detection of earth faults. An MCB should be fitted to protect against overloads.

The filters of the PSU's are normally electrically symetrical so the leakage currents to earth from each of the two phases to earth will be equal and opposite and so there will be no in-balance on the phases at the RCD so it will not trip due to the filters.

The same can be achieved with a 230 to 110 centre tapped "site safety" isolation transformer providing 55-0-55 if all the PSUs will work on 110 volt. But the cabling may have to upgraded to carry the current which will double that of a 230 volt supply.

 

[Spark123]

If going for a centre tap arrangement it is essential that all phases have overcurrent protection placed in them, I have seen the mess it makes if you don't!

 

[bernardgreen]

If going for a centre tap arrangement it is essential that all phases have overcurrent protection placed in them, I have seen the mess it makes if you don't!

I agree

In theory any over current faults phase to phase will involve both phases taking over current and will be protected by a single MCB in either phase.

Phase to earth ( centre tap ) faults would be detected and protected by the RCB.

But thinking out out of the box using two MCBs, one on each phase would be better.

 

[JohnD]

surely a DP MCB would be even better?

 

[Spark123]

We tend to use fuses so isn't possible to cut both line condutors at the same time.