Hi Adie, You obviously have a Metrel 61577 Instaltest where rotary switch position 10 is the RCD No-trip, Earth Fault Loop Resistance (actually impedance but Metrel call it resistance) mode (marked U[SUB]c[/SUB],R[SUB]l[/SUB]). There is no reason to buy a new MFT to carry out No-trip loop measurements. The 61577 is quite capable of accurate results when the looP function is selected. The difference between the 61577 and some modern MFTs is the relative speed of the test, not the accuracy. In fact the 61577 is likely to be more accurate than some of the modern speedsters which can be severely degraded by electrical noise.
Test Method:
1) Set function switch to Uc, RL position, “Contact Voltage initial menu” or “Fault Loop Resistance initial menu” will be displayed; Uc or looP is displayed for a while before initial menu appears.
Note:- Initially the MFT display shows either the Contact Voltage (U[SUB]c[/SUB] ) or Fault Loop Resistance (R[SUB]l[/SUB] ) menu depending on which menu was last used.
2) Select required function i.e. Contact Voltage or Fault Loop Resistance by pressing the SELECT key first. The previously set function (Uc or looP message) starts to flash. Select the function by using the ↑ and ↓ keys.
Note:- Fault Loop Resistance function takes longer measurement time but offers better accuracy of fault loop resistance in comparison with the RL sub-result in Uc function.
3) Select Nominal differential current I∆n as follows:
Press the SELECT key, last set Nominal differential current starts to blink.
Select appropriate value by using the ↑ and ↓ keys. The following values are available to be selected: 10, 30, 100, 300, 500 or 1000 mA.
4) Select Type of RCD involved as follows:
Press the SELECT key after setting Nominal differential current, last set type starts to blink.
Select appropriate type using the ↑ and ↓ keys. Standard (General) RCD type (G is displayed) or Selective (time-delayed) RCD type (S is displayed) can be selected.
5) Select Contact Voltage Limit (Ulim) as follows:
Press the SELECT key after setting RCD type, last set Limit Contact Voltage starts to blink.
Select appropriate value using the ↑ and ↓ keys. 50V (Most of BS7671) or 25V (BS7671 710 Medical Locations only) can be selected.
Press the SELECT key again after selecting the Limit Contact Voltage, to enter “Contact Voltage” or “Fault Loop Resistance” initial menu.
6) Connect test cable to the tested object (mains outlet or other test terminals). Mains plug mark will be displayed as soon as mains voltage appears at test terminals.
7) If Uc function is selected:
Press the START key and release it. Measurement will be carried out and result (Uc at I∆N - standard RCD or Uc at 2I∆N - selective RCD) will be displayed.
Check Fault Loop Resistance sub-result and preset the Contact voltage limit by using the DISPLAY key. Store the displayed result for documentation purpose if required.
8) If looP function is selected:
Press the START key and release it, the measurement will start to run. Three dots (. . .) will flash until the measurement is finished, then result is displayed.
The measurement may take longer (up to 1 min approx) in order to ensure a more accurate and stable test result in comparison with the RL sub-result in Uc function.
Check Contact Voltage sub-result (Uc at I∆N – standard (G) RCD or Uc at 2I∆N – selective (S) RCD) and preset the Contact voltage limit by using the DISPLAY key. Store the displayed result for documentation purpose if required.
