Construction sites and the use of RLV, guidance on the use of residual protection?

Question

RCDs on 110v sockets 
 from the discussion above, which is a few years ago, a conclusion was made that RCD&rsquo;s are not required for RLV/ CTE transformer used in construction sites from Sec 4. It is clear from section 4 that additional protection for RLV sockets is not a requirement, for obvious reasons CTE 55v etc. 
 however I have noticed a more recent trend of RCCB&rsquo;s integral to the secondary side of the windings being implemented to ensure fault protection that can clear within the 5 second disconnection time under 411.8.3. It would seem primarily this is due to chaining multiple extension leads together or excessive lighting circuit lengths, which are difficult to account for except by direct supervision on site. Blakley, being one of the leading manufacturers offer a great deal of good technical guidance on the subject - but largely it would appear the practice of having both RLV + residual protection combined into one transformer is down to preference rather than a requirement of BS7671 or BS7375 for that matter, meaning this would be &lsquo;best practice&rsquo; as apposed to mandatory unless a contractor were to make part of their policy. I would be interested in the forums opinions on this subject, and your own experiences, I think there are changing trends in the industry as to what was historically an acceptable risk may no longer be the case, particularly I&rsquo;ve noticed 110v Tx plugged into blue commando sockets with a 30mA RCD protecting the primary side, I&rsquo;m unsure what degree of protection this will offer to outgoing artic flex on the secondary side and it doesn&rsquo;t seem it would be the same as a multiple pole device that&rsquo;s installed on the secondary side?

AMK · Accepted Answer

Systems operating at reduced low voltage are required to comply with the disconnection provisions outlined in section 411.8.3. In instances where compliance with the disconnection times specified in table 41.6 is not feasible, the installation of a Residual Current Device is necessary to ADS

broadgage · Answer

From a practical point of view, there is little need for RCD protection on 110 volt center tapped supplies. Without an RCD, disconnection times in the event of earth faults are likely to exceed those permitted for 230 volt mains circuits. 
 But does this matter at the much lower voltage ? On a 230 volt mains circuit, presuming similar sized live cores and CPC conductors, then during an earth fault the case of any class one appliance will be live at 115 volts, less risky than full mains voltage, but still potentially dangerous. Hence the need for RCD protection in order to promptly disconnect such faults. 
 On a system with only 55 volts to earth, then the touch voltage on a class one appliance during an earth fault will be only 27.5 volts, which is very unlikely to be dangerous, and prompt disconnection therefore a lower priority. 
 It is increasingly common to provide RCD protection on 55/110 volt circuits, and this does no harm, but the actual need is debateable. 
 An RCD on the 230 volt input to a site transformer will give NO PROTECTION to the 110 volt output. Anyone who doubts this should try a simple experiment. Plug in the transformer to the RCD 230 volt socket. Connect a test lamp between either pole of the output and the protective earth. Does the input RCD trip ? 
 An input RCD is a wise precaution against a damaged mains lead or plug or transformer but gives no protection to the output.

AMK · Answer

Is it the formula when In the absence of specific data regarding the transformer&rsquo;s percentage impedance, it is permissible to employ a factor of 1.25 in the calculations of the secondary earth fault loop impedance (Zsec). The earth loop impedance should also fulfil Zs&times; Ia&le;Uo requirements with either Ia being the current causing instant disconnection for the circuit breaker or RCD.

AMK · Answer

when it comes to ensuring compliance with the maximum permitted Zs values, especially in scenarios where cable lengths are unknown or the stability of the earthing system is uncertain, we must be proactive.

gkenyon · Answer

TaZ10101 said: particularly I’ve noticed 110v Tx plugged into blue commando sockets with a 30mA RCD protecting the primary side, I’m unsure what degree of protection this will offer to outgoing artic flex on the secondary side and it doesn’t seem it would be the same as a multiple pole device that’s installed on the secondary side? 
 It will have no effect on the secondary side for a fault to Earth on the secondary. 
 For an RCD to be used for ADS on the secondary of the transformer in the RLV system, it needs to be located on the secondary.

Sparkingchip · Answer

I have been in the construction industry for fifty years and still have a full kit of 110 volt tools, extension leads, four transformers and three generators. 
 I have only ever blown fuses due to overload back in the days of fused extension plugs on extension leads, not due to faults and all three of the petrol engine generators I have will stall before the circuit protection built into them operates, as I have found out. 
 None of the younger guys have 110 volt kit, they are using battery tools, the voltages and risks are probably similar. I don't think I would want to short one of the big backpack batteries used on some of the corded "cordless" power tools, particularly with the battery pack on my back.

AJJewsbury · Answer

much debate remains especially with smaller gensets, stuff like - less than 5kva doesn&rsquo;t need any earthing, is what you&rsquo;ll often hear. This is only partially correct, the code states the task should be short duration with only 1 item of class II equipment in use - but this is a pretty broad statement, and doesn&rsquo;t say if the generator output should have a RCD or not - often they don&rsquo;t. 
 
 Unearthed, the generator would usually meet the requirements for a separated system - when feeding just one item of current using equipment (possibly better read as a system having at most one exposed-conductive-part), the principles are covered by BS 7671 section 413 - generally the system remains perfectly safe on faults and ADS of any kind isn't required. RCDs would be pretty pointless as the system isn't referenced to Earth. Feeding two or more items of current using equipment (or rather having two or more exposed-conductive-parts) is slightly riskier and is covered by BS 7671 418.3 - while it remains safe on 1st faults, a 2nd fault to a different exposed-conductive-part from a different line, raises the possibility of a shock between the two exposed-conductive-parts. Again RCDs wouldn't help, unless the two exposed-conductive-parts were fed from two separate RCDs - a single common RCD wouldn't see any imbalance. Generally ADS for 2nd faults is handled by connecting all the exposed-conductive-parts together (not to Earth) and using overcurrent devices to disconnect. 
 - Andy.

Construction sites and the use of RLV, guidance on the use of residual protection?

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